Metallurgical apparatus – Process – Cooling
Reexamination Certificate
2001-10-24
2004-05-11
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Process
Cooling
C266S113000, C266S114000, C148S333000
Reexamination Certificate
active
06733720
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus and a method for cooling a hot rolled steel strip having a high temperature and a method for manufacturing the hot rolled steel strip.
DESCRIPTION OF THE RELATED ARTS
In general, a hot rolled steel strip is manufactured in a step where a slab is heated to the specified temperature in a heating furnace and is rolled to the required thickness by a rough rolling mill to form a rough bar, and finally the resultant bar is rolled by a continuous hot rolling mill having plural rolling stands. The hot rolled steel strip is cooled at a cooling stand on a runout table and then is coiled by a coiler.
An online cooling apparatus to transfer as rolled high temperature steel strip and to continuously cool before coiling by the coiler should be first designed to consider steel strip transferring ability.
For example, for cooling an upper surface of the steel strip, circular laminar cooling nozzles can be provided at an upper area of the steel strip transfer roll (called a roller table) and at a straight line over the width of the steel strip for ejecting plural laminar cooling water. The runout table comprises plural transfer rolls.
At this time, laminar nozzles with the same length as an axial length of the transfer roll is arranged just above the roll to prevent a steel strip path line from lowering below a line connecting upper contact points of the transfer roll even when pressing the steel strip by water pressure of the falling down cooling water. In addition, spray nozzles are arranged between transfer rolls to eject cooling water upward for cooling the lower surface of the steel strip.
Therefore, this cooling mode does not ensure an exact symmetrical cooling for the upper and lower surface of the steel strip, resulting in intermittent cooling especially at the upper surface of the steel strip. This makes a rapid cooling (for example, cooling speed of 200° C./sec or more for 3 mm in sheet thickness) impossible practically.
Recently, the rapid (strong) cooling, however, has been required to produce the hot rolled steel strip with fine grain size because of excellent machinability and to manufacture low Ceq high strength product.
Upon rapid cooling of the hot rolled steel strip, the conventional cooling apparatus has been involved in the following problems.
At rapid cooling, a cooling start point is different at the upper and lower surfaces of the steel strip, which causes to generate non-uniformity in material property. After cooling, cooling water remains at the upper surface of the steel strip to cause excessive cooling at the upper surface. The excessive cooling is not uniform in a longitudinal direction, resulting in variation in cooling finish temperature in this direction.
In the width direction, cooling water tends to flow from sides of the steel strip to both line sides to cause excessive cooling at the end compared with the center of the strip, fluctuating the temperature finish time. This makes material property non-uniform.
Hence, a water breaking method has been proposed such as a method to eject fluid in slant direction across the steel strip to discharge cooling water JP-A-9-141322, (the term “JP-A” referred to herein signifies “Unexamined Japanese Patent Publication”) or a method using a restriction roll (called a pinch roll) as a water block roll to interrupt cooling water, JP-A-10-166023.
However, the former method when applying strong cooling is useless because a large amount of cooling water remains on the steel strip. In the latter method, a top of the steel strip is left at a free state during transfer at the interval from the final rolling mill to the coiler, the strip passes at non-restrained state moving up and down in waving action.
As a result, the restriction roll if provided at the roller table disturbs safe passing of the strip, which is difficult to apply the roll as the cooling apparatus for the runout. Strong cooling if applied at the top of the vibrating steel strip at non-restricted state will further escalate vibration of the top end of the steel strip unavoidably to damage due to contact with the restriction roll.
On the other hand, JP-A-6-328117 proposes an effective cooling method by increasing cooling water at the steel strip top end for the lower surface than that for the upper surface. Change in the cooling water ratio, however, will unbalance the cooling effect to upper and lower surfaces especially to make unavoidably material property non-uniform. In addition, the strong cooling necessary for changing in material property is difficult because of insufficient cooling at the lower surface.
In particular, for cooling so called thinner sheet less than 2 mm in thickness, the steel strip top vibrates up and down by cooling water pressure or the steel strip tends to fold at the last half of the runout table to disturb stable passing, finally stopping the steel strip passage.
In JP-B-59-50420, (the term “JP-B” referred to herein signifies “Examined Japanese Patent Publication”) a cooling water guide is arranged between plural roller tables in the frame provided in the feeding direction of the steel strip. To maintain the specified interval between the guide and steel strip surface, a press machine for the steel strip is disclosed by installing a guide roll at the guide.
This machine, however, is difficult to hold uniform interval between the cooling water guide and the steel strip surface because the steel strip top is transferred waving up and down. This method if applied for a thinner steel strip causes sticking trouble because of disturbing smooth passage at touching the steel strip top to the transfer roll.
The steel strip usually is not flat with an edge waving or center buckling. Such steel strip failed in its shape cannot be pressed by the guide roll, resulting in another leveler provided for flat shape to escalate working man-hour.
JP-B-4-11608, discloses a direct cooling apparatus to cool the steel strip just after delivering from the roll mill. But this apparatus is not available for installing a detecting sensor for steel strip temperature and sheet thickness during rolling step as significant items in quality control of the steel strip.
This requires an air cooling space after the final finishing mill to install a thermometer or a thickness gage at the space. However, cooling is difficult to start at the steel strip top end, because it vibrates up and down at free state.
While, JP-U-57-82407 discloses a technique giving a travel driving force to the steel strip by providing another driving roll which can rotates upwards to the table roll.
This technique, however, should arrange an upper driving roll as densely as the lower table roll. If not, the steel strip top end might be crashed into the roll clearance or be broken at the half way the steel strip top end once crashed into the upper or lower rolls generates up and down vibration due to reaction force to disturb stable passage, especially for thinner strip. Rolls if arranged densely at both upper and lower sides will disturb strong cooling because the cooling nozzle area is narrowed.
SUMMARY OF THE INVENTION
It is an object of the first invention to provide an apparatus and a method for cooling a hot rolled steel strip wherein the steel strip having no tension is cooled stably and strongly at a runout table arranged between a finishing mill and a coiler.
It is an object of the second invention to provide an apparatus and a method for cooling a hot rolled steel strip wherein cooling water is removed rapidly from the surface of the steel strip during cooling the steel strip, to move the steel strip smoothly and to produce the hot rolled steel strip without any defect.
It is an object of the third invention to provide an apparatus and a method for cooling a hot rolled steel strip wherein a top end of a steel strip moves smoothly from a final finishing mill to a coiler to cool the steel strip rapidly and to ensure a cooling efficiency.
It is an object of the fourth invention to provide a method for manufacturing a hot roll
Fujibayashi Akio
Hino Yoshimichi
Ikemune Shozo
Imada Sadanori
Minote Toru
Frishauf Holtz Goodman & Chick P.C.
Kastler Scott
NKK Corporation
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