Patent Application
20090211670
In general, a hot rolled steel strip is manufactured by heating a slab to a predetermined temperature in a reheating furnace, hot rolling the heated slab into a sheet bar having a predetermined thickness using a roughing mill, hot rolling the sheet bar into a steel strip having a predetermined thickness using a finishing mill having a plurality of rolling stands, transferring and cooling the hot rolled steel strip on a run-out table using a cooling apparatus, and then coiling the steel strip on a coiler. The run-out table is a transfer apparatus provided downstream of the finishing mill to transfer the hot rolled steel strip on a plurality of transfer rollers disposed at a suitable pitch.
A conventional cooling apparatus provided on the run-out table is so contrived as to mainly aim stable transfer of steel strip, as typically shown in
Recently, excellent workability, high strength with low carbon equivalent and the like have been required for a hot rolled steel strip. For these requirements, grain refining of steel strip is effective, and thus the steel strip need to be more rapidly cooled after hot rolling. In particular, the steel strip having low carbon equivalent such as an ultra low carbon steel strip should be cooled at a cooling rate exceeding 200° C./s because austenitic grains after hot rolling tend to become coarse due to recrystallization.
To conduct such rapid cooling, Japanese Unexamined Patent Application Publication No. 62-259610 discloses a method for increasing cooling capability for bottom surface of steel strip using a bottom surface cooling apparatus where cooling water jetting plates having a plurality of holes are disposed between transfer rollers and also function as a guide, and jetting cooling water toward the steel strip through the holes at different angles.
However, the method described in Japanese Unexamined Patent Application Publication No. 62-259610 causes various problems as follows.
(1) A hot rolled steel strip undulates vertically while being transferred on a run-out table when the leading end of the hot rolled steel strip lies between a finishing mill and a coiler, because the hot rolled steel strip is not under any tension. Cooling of such a tension free steel strip in this method causes further vertical waves. As a result, a sufficient volume of cooling water is not applied and it is impossible to cool, for example, a steel strip of 3 mm in thickness at a cooling rate exceeding 200° C./s.
(2) This method does not enable the top and bottom surfaces of the steel strip to be cooled at the same cooling rate.
(3) This method presupposes cooling at a water flow rate of about 1,000 L/min·m2, but a higher water flow rate is required to cool a steel strip of, for example, about 3 mm in thickness at a cooling rate exceeding 200° C./s. In the cooling apparatus used n this method, as shown schematically in
Widening the space between the cooling water jetting plate and the steel strip, as shown in
When the space between the cooling water jetting plate functioning also as a guide and the steel strip is arranged properly, the temperature profile in the width direction of the steel strip after cooling shows an M shape which is the sum of the inverted-V shape in
(4) According to this method, when the cooling water is jetted toward the steel strip at different angles from a plurality of holes in the cooling water jetting plate functioning as nozzles, the distance that the cooling water travels varies depending on the nozzles. The cooling water jetted aslant to the steel strip travels a longer distance, thus greatly reducing the flow velocity to fail to efficiently cool the steel strip. As described in (3), cooling capability is greatly affected by the jetted cooling water, so it is more difficult to uniformly cool the steel strip in the width direction.
An object of the present invention is to provide a cooling apparatus for hot rolled steel strip which stably transfers a hot rolled steel strip and cools it rapidly and uniformly after hot rolling, a manufacturing method and a production line for hot rolled steel strip using such a cooling apparatus.
The above-mentioned object is accomplished by a cooling apparatus for hot rolled steel strip comprising: top surface cooling means provided above a hot rolled steel strip transferred with transfer rollers after hot rolling to cool the top surface of the hot rolled steel strip; and bottom surface cooling means provided below the hot rolled steel strip to cool the bottom surface of the hot rolled steel strip, wherein each of the top surface cooling means and the bottom surface cooling means comprises: a protective member disposed close to the surface of the hot rolled steel strip and having at least one cooling water passage hole; at least one cooling water header opposing the hot rolled steel strip separated by the protective member; and cooling water jetting nozzles protruding from the cooling water header and jetting cooling water approximately vertically toward the surface of the hot rolled steel strip through the cooling water passage hole, the tips of the cooling water jetting nozzles being disposed farther from the hot rolled steel strip than the surface, opposing the hot rolled steel strip, of the protective member.
When such a cooling apparatus for hot rolled steel strip is provided on a run-out table in a production line for hot rolled steel strip, hot rolled steel strip can be transferred stably, and cooled rapidly and uniformly.
The production line includes a roughing mill 1 to roll a slab into a sheet bar 2, a finishing mill 3 including a plurality of rolling stands to roll the sheet bar 2 into a hot rolled steel strip 9 having a predetermined thickness, a run-out table 5 to transfer the hot rolled steel strip 9 after hot rolling on transfer rollers 7, and a coiler 6 to coil the hot rolled steel strip 9. The run-out table 5 is provided, just downstream of the finishing mill 3, with a cooling apparatus 4 according to the present invention to rapidly cool the hot rolled steel strip 9. In addition, the conventional cooling apparatus 8 shown in
The cooling apparatus for hot rolled steel strip according to the present invention includes bottom surface cooling means 4a provided below a hot rolled steel strip 9 to cool the bottom surface of the hot rolled steel strip 9 and top surface cooling means 4b provided above the hot rolled steel strip 9 to cool the top surface of the hot rolled steel strip 9.
Each of the cooling means 4a and 4b is provided with protective member plates 10, consisting of bottom protective members 10a and top protective members 10b, disposed close and approximately parallel to the surface of the hot rolled steel strip 9, and cooling water headers 12, consisting of bottom surface cooling water headers 12a and top surface cooling water headers 12b, disposed to oppose the hot rolled steel strip 9 separated by the protective members 10a or 10b. Each of the cooling water headers 12a or 12b is provided with protruding cooling water jetting nozzles 15 at a suitable pitch in the width and longitudinal directions of a run-out table. The tips of the cooling water jetting nozzles 15 are disposed farther from the hot rolled steel strip 9 than the surfaces, opposing the hot rolled steel strip 9, of the protective members 10. Furthermore, each of the protective members 10 has a plurality of cooling water passage holes 11 to pass cooling water. Through the cooling water passage holes 11, each of the cooling water jetting nozzles 15 jets cooling water approximately vertically toward the surface of steel strip.
Two guide rollers 14 are provided above the hot rolled steel strip 9 approximately opposing the transfer rollers 7 provided under the hot rolled steel strip 9. The guide rollers 14 allow to transfer the hot rolled steel strip 9 more stably. Preferably, the guide rollers 14 are provided at least one position above the hot rolled steel strip 9 approximately opposing the transfer rollers 7. The guide rollers 14 may be provided at all the positions approximately opposing the transfer rollers 7.
The top surface protective members 10b of the top surface cooling means 4b are disposed close to the surface of steel strip at positions other than where the guide rollers 14 are provided.
On the other hand, the bottom surface protective members 10a of the bottom surface cooling means 4a are disposed between the transfer rollers 7 provided in the longitudinal direction of the run-out table at a suitable pitch. Therefore, the cooling water jetting nozzles 15 of the bottom surface cooling water headers 12a are disposed between the transfer rollers 7. In
With regard to the top surface cooling water headers 12b, the same effect is achieved. Preferably, the top surface cooling water headers 12b are arranged to oppose the bottom surface cooling water headers 12a separated by the hot rolled steel strip 9. This provides the following advantages: The top and bottom cooling can be easily balanced; the positions of the headers to start cooling the top and bottom surfaces can be easily adjusted; the hot rolled steel strip 9 can be stably transferred due to the water pressure from the upside and downside.
Preferably, each of the cooling water jetting nozzles 15 of the top surface cooling means 4b protruding from each of the top cooling water headers 12 is arranged to approximately oppose each of the cooling water jetting nozzles 15 of the bottom surface cooling means 4a protruding from each of the bottom cooling water headers 12 separated by the hot rolled steel strip 9. This is effective to bring the cooling of the top and bottom surfaces and the water pressure thereof into balance.
As described above, each of the cooling water jetting nozzles 15 protrudes from each of the cooling water headers 12 and is disposed so as to jet cooling water approximately vertical to the surface of the steel strip. In other words, when nozzle installation surfaces of the cooling water headers 12 are parallel to the steel strip as shown in
Laminar nozzles are generally used as the cooling water jetting nozzles 15. Since the cooling water jetting outlets of laminar nozzles are cylindrical, jetted water flow collides with the steel strip 9 as laminar flow without divergence. Here, the cylindrical laminar flow is primarily laminar flow but it may contain some turbulent flow.
In the cylindrical laminar flow, the water flow reaches the steel strip without divergence to give good cooling efficiency, resulting in rapid cooling at a rate exceeding 200° C./s. On the other hand, in the non-laminar flow, since the flow velocity of the cooling water jetted from nozzles is reduced by cooling water remaining between the steel strip and the nozzles, even if the nozzles are disposed close to the steel strip, the cooling efficiency is low.
The conventional cooling apparatus uses laminar flow cooling nozzles for cooling the top surface of steel strip. However, since the main cooling is carried out by film boiling in which cooling water is poured over the entire steel strip to cover its surface with cooling water, the cooling rate is 100° C./s at highest. On the other hand, the cooling apparatus according to the present invention uses laminar nozzles as cooling water jetting nozzles as the conventional cooling apparatus, but the cooling apparatus according to the present invention can jet a large amount of cooling water at a water flow rate exceeding about 2,500 L/min·m2. As a result, the cooling water covers the entire steel strip and also the cooling water jetted from the nozzles is directly applied to the steel strip, making it possible to cool the steel strip of about 3 mm in thickness at a cooling rate exceeding 200° C./s. The cooling rate depends on the thickness of steel strip and increases as the thickness becomes thinner. When a cooling condition such as the water flow rate is constant, the product of the strip thickness and the cooling rate is almost constant. Accordingly, even when the strip is thick, the desired cooling rate can be achieved, for example, by increasing the water flow rate.
The diameter of the cooling water jetting nozzles of the present invention is preferably 1 to 10 mm. When the diameter is smaller than 1 mm, it is difficult to generate the cylindrical laminar flow. Since the cooling using the cooling apparatus according to the present invention needs collision pressure, the flow velocity at nozzle outlets is constant and the amount of water increases with increasing diameter of jetting outlets. However, since cooling capability is saturated at a certain amount of water, the jetting outlet diameter should be 10 mm or less from an economic standpoint.
The above-mentioned protective members disposed between cooling water headers and steel strip play two roles of stably transferring the steel strip and protecting the cooling water headers and the cooling water jetting nozzles from collision with the steel strip. The cooling water passage holes in the protective members function not only as jetting holes of cooling water and but as drain holes of jetted cooling water.
Each of the protective members provided with cooling water passage holes may be, for example, a flat plate having slits shown in
When flat plates shown
Each of the slit shaped cooling water passage holes 11 of the protective members 10 is provided with a plurality of cooling water jetting nozzles 15 to jet cooling water as the laminar flow 13. The orifices of the slit shaped cooling water passage holes 11 are preferably as large as possible to drain jetted cooling water, but larger orifices cause collision of the leading end of the steel strip 9 with the slit edge resulting in seizing and damage. Accordingly, the size of an orifice of the slit shaped cooling water passage holes 11 is preferably large enough to hold about two to ten cooling water jetting nozzles 15 in a line, as shown in
As shown in
Preferably, the longitudinal direction of the slit shaped cooling water passage holes 11 inclines in the horizontal plane with respect to the transferring direction of the steel strip 9 in order to allow easy drainage to the outside of the cooling apparatus. When the longitudinal direction of the slit shaped cooling water passage holes 11 is perpendicular to the transferring direction of the steel strip 9, it may disturb the flow of the drainage or may cause collision of the leading end of the steel strip 9 with the slit shaped holes giving damage to the steel strip 9 and the cooling water passage holes 11. When the longitudinal direction of the slit shaped cooling water passage holes 11 is parallel to the transferring direction of the steel strip 9, the flow of the drainage is not smooth. As shown in FIG. 8A, the slit shaped cooling water passage holes 11 are disposed so as to be almost axisymmetric to the central line of the run-out table and the longitudinal direction of the cooling water passage holes 11 inclines in the horizontal plane to diverge toward the transferring direction of the steel strip 9. This is more preferable for the smooth flaw of the drainage to the outside of the cooling apparatus.
In this example, the thickness of the protective members 10a is small, and tips 16 of the cooling water jetting nozzles 15 are disposed below the bottom surface of the protective members 10a.
In this example, the thickness of the protective members 10a is large, and tips 16 of the cooling water jetting nozzles 15 are disposed inside the cooling water passage holes 11 of the protective members 10a.
In the bottom surface cooling means shown in
First, the impinging velocity of the laminar flow 13 of cooling water to the steel strip and the pitch between the cooling water jetting nozzles 15 are determined so as to achieve a desired cooling rate.
Then, the distance Xa from the tips 16 of the cooling water jetting nozzles to the surface of the steel strip is determined to secure the impinging velocity in view of the diameter of the cooling water jetting nozzles 15. It is preferable that the distance Xa from the tips 16 of the cooling water jetting nozzles to the surface of the steel strip be 100 mm or less. When the cooling water used for cooling the steel strip 9 flows out from the space between the steel strip 9 and the protective members 10a, the cooling water prevents the laminar flow 13 of the cooling water jetted from the cooling water jetting nozzles 15 from colliding with the steel strip. In particular, when the distance Xa exceeds 100 mm, the flow velocity of the laminar flow 13 of the cooling water significantly decreases. This further disturbs the collision of the cooling water with the steel strip, failing in rapid cooling. As described above, the tips 16 of the cooling water jetting nozzles are disposed farther from the steel strip 9 than the surface, opposing the steel strip 9, of the protective members 10a. In other words, the distance Xa from the tips 16 of the cooling water jetting nozzles to the surface of the steel strip is determined to be longer than the distance Ya, which will be described below, from the top surfaces of the protective members 10a to the surface of the steel strip.
The distance Ya from the top surfaces of the protective members 10a to the surface of the steel strip is determined in view of stably transferring the steel strip 9 above the top surfaces of the protective members 10a. When the protective members 10a are disposed at the lower positions, as shown in
The distance Za from the bottom surfaces of the protective members 10a to the cooling water headers 12a yields a necessary space for rapidly draining the cooling water jetted from the cooling water jetting nozzles 15, and thus the Za is preferably larger. However, when the Za is too large, the cooling water jetting nozzles 15 protruding from the cooling water headers 12a must be significantly long. On the other hand, the ratio of the diameter of the cooling water jetting nozzle to the length of a straight run of the cylindrical laminar nozzle used in the cooling water jetting nozzles 15 is preferably 5 to 20. The ratio over 20 increases the flow resistance, and thus the supply pressure of the cooling water should be increased, which is not economical. When the ratio is less than 5, the cooling water is jetted in non-laminar flow as shown in
The distance Xb from the tips 16 of the cooling water jetting nozzles to the surface of the steel strip 9, the distance Yb from the bottom surfaces of the protective members 10b to the surface of the steel strip, and the distance Zb from the top surfaces of the protective members 10b to the cooling water headers 12b are determined as follows.
The distance Xb from the tips 16 of the cooling water jetting nozzles to the surface of the steel strip in the top surface cooling means corresponds to the distance Xa in the bottom surface cooling means described above. In the top surface cooling means, since the cooling water remains on the steel strip 9, the distance is determined in additional view of the number and position of the guide rollers 14, the distance Yb between the bottom surfaces of the protective members 10b and the surface of the steel strip, and the thickness of the protective members 10b. Here, the distance Xb from the tips 16 of the cooling water jetting nozzles to the surface of the steel strip is preferably 100 mm or less, similar to the distance Xa in the bottom surface cooling means.
The distance Yb from the bottom surfaces of the protective members 10b to the surface of the steel strip corresponds to the distance Ya in the bottom surface cooling means described above and is preferably 10 to 50 mm, as in the bottom surface cooling means.
The distance Zb from the top surfaces of the protective members 10b to the cooling water headers 12b corresponds to the distance Za in the bottom surface cooling means and is determined in additional view of the number and position of the guide rollers 14 and the space between the guide rollers 14 and the steel strip 9. The area dimension of the cooling water passage holes 11 of the protective members 10b is also determined in view of the number and position of the guide rollers 14 and the space between the guide rollers 14 and the steel strip 9.
As shown in
In the bottom surface cooling means, the cooling water jetted to the steel strip 9 flows down due to gravity through the cooling water passage holes 11 in the protective members 10a. On the other hand, in the top surface cooling means, the majority of the jetted cooling water is drained from both edges in the width direction. Therefore, the cooling water that is not drained from the space between the steel strip 9 and the protective members 10b flows into the space between the protective members 10b and the cooling water headers 12b from below the protective members 10b through the cooling water passage holes 11. Consequently, the tips 16 of the cooling water jetting nozzles 15 are preferably disposed inside the cooling water passage holes 11 so that the flow of the cooling water jetted from the cooling water jetting nozzles 15 is not affected by the drained water flowing toward both edges in the width direction in the space above the protective members 10b.
In the bottom surface cooling means, since the flow of the jetted cooling water may be affected by the drained water flowing toward both edges in the width direction in the space between the cooling water headers 12a and the protective members 10a depending on the amount of the drained water, the tips 16 of the cooling water jetting nozzles 15 are preferably disposed inside the cooling water passage holes 11 of the protective members 10b.
The guide rollers 14 provided above the transferred hot rolled steel strip 9 preferably has a gap about 5 mm from the surface of the hot rolled steel strip 9, when no problems, such as jamming of the leading end of the steel strip 9 or looping of the steel strip 9, occur during transfer. When the above-mentioned problems occur during transfer, the gap between the guide rollers 14 and the steel strip 9 is broadened so as not to raise the loop and to send the leading and trailing ends of the steel strip out of the cooling means. When the broadened gap between the guide rollers 14 and the steel strip 9 disturbs the drainage, a pinch roll is preferably provided at least one position of the entry side, the delivery side, or between both sides of the cooling means to forcibly pinch the steel strip 9 and send it into or out the cooling means.
The above-mentioned cooling apparatus for hot rolled steel strip according to the present invention can almost uniformly jet the cooling water from above and below and rapidly cool the hot rolled steel strip while stable transfer of the steel strip is maintained by the protective members and the guide rollers. Since the cooling water jetted to the surface of the steel strip is properly drained and the influence of jetted cooling water flow is minimized to cool the hot rolled steel strip, rapid and uniform cooling in the width direction can be achieved.
As shown
Using a production line for hot rolled steel strip shown in
As shown in
One bottom surface cooling water header 12a is disposed between two adjacent transfer rollers. The bottom surface cooling water headers 12a are provided with the cooling water jetting nozzles 15 used for jetting cooling water at the same pitch in both the width and the longitudinal directions. Laminar nozzles are used as the cooling water jetting nozzles 15.
The distance Xa between the surface of the steel strip and the tips 16 of the cooling water jetting nozzles is 25 mm, the distance Ya between the surface of the steel strip and the top surfaces of the bottom surface protective member plates 10a is 10 mm, and the distance Za between the bottom surface protective member plates 10a and the cooling water headers 12a is 30 mm.
Top surface cooling means 4b comprises three guide rollers 14 which are disposed to oppose the transfer rollers 7 and have a space of 5 mm from the steel strip 9, top surface protective member plates 10b having a thickness of 25 mm which are disposed close and parallel to the surface of the transferred hot rolled steel strip 9, a plurality of cooling water passage holes 11 in the top surface protective member plates 10b as passages for cooling water, cooling water jetting nozzles 15 having outlets with a diameter of 5 mm, of which the tips are disposed higher than the bottom surfaces of the protective member plates, and top surface cooling water headers 12b from which the cooling water jetting nozzles 15 protrude.
The top surface cooling water headers 12b are disposed to oppose the cooling water headers 12a of the bottom surface cooling means. The top surface cooling water headers 12b are provided with the cooling water jetting nozzles 15 used for jetting cooling water at a pitch of 30 mm in the width direction and at a pitch of 30 mm in the longitudinal direction. Laminar nozzles are used as the cooling water jetting nozzles 15.
The distance Xb between the surface of the steel strip and the tips 16 of the cooling water jetting nozzles is 30 mm, the distance Yb between the surface of the steel strip and the bottom surfaces of the top surface protective member plates 10b is 15 mm, and the distance Zb between the top surface protective member plates 10b and the top surface cooling water headers 12b is 30 mm.
As a comparative example, a similar test was carried out using a production line provided with a cooling apparatus for hot rolled steel strip shown in
The cooling apparatus used in the comparative example has almost the same constitution as the cooling apparatus of the present invention shown in
When the cooling apparatus for hot rolled steel strip according to the present invention is used, the temperature profile in the width direction of the steel strip is around ±20° C., and almost uniform cooling in the width direction is achieved. In addition, the variation in strength of the hot rolled steel strip in the width direction is 20 MPa.
In contrast, in the comparative example, the temperature profile in the width direction of the steel strip is ±50° C. or more and shows the V-shaped profile in the width direction. Because of high temperature at both edges in the width direction of the steel strip, the steel strip is deformed and is not coiled normally. The variation in strength of the hot rolled steel strip in the width direction is 80 MPa.
When the protective member plates of the cooling apparatus used in the comparative example are disposed close to the steel strip, the temperature profile shows the inverted-V shape in the width direction of the steel strip.
This application is a Continuation application of application Ser. No. 10/508,029 filed Apr. 6, 2005, which is the United States national phase application of International application PCT/JP02/08113 filed Aug. 8, 2002. The entire contents of each of application Ser. No. 10/508,029 and PCT/JP02/08113 are hereby incorporated by reference herein. The present invention relates to a cooling apparatus for hot rolled steel strip, a manufacturing method for hot rolled steel strip and a production line for hot rolled steel strip using the cooling apparatus.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10508029 | Apr 2005 | US |
| Child | 12380254 | US |
