The invention relates to the general field of controlled cooling of hot rolled metal and specifically to the accelerated cooling and direct quenching of steel strips and plates.
The controlled cooling of hot rolled steel is very important for achieving the desired microstructure and properties of the steel product. Modern plate and hot strip mills generally use powerful cooling systems for this purpose whereby the accurate control of the temperature and the cooling rate are very important. Water is usually used as the cooling fluid.
One of the most successful designs for an apparatus for controlled cooling is a design with a number of top cooling headers and bottom cooling headers, through which cooling water jets are being sprayed onto the top surface and the underside of the hot rolled metal plate or strip, as described for example in EP0178281.
A simplified cross-section of this type of apparatus is illustrated in
In order to achieve good flatness of the cooled plates or strips it is very important that the top and bottom cooling are applied at the same time and that the top and bottom cooling are equal. If one surface is cooled before the other then the plate will tend to bow and to form either a gutter or inverted gutter shape or develop another type of flatness problem. In order to ensure that the cooling of the top surface starts and stops at the same time as the cooling of the bottom surface it is common practice to use fluid flows in the form of jets or streams, which are called zone separation sprays and are applied from zone separation spray devices. The zone separation sprays constrain the cooling water on the top surface of the plate or strip to a desired area, in the above example to the area between zone separation spray devices 10 and 11. The zone separation sprays are depicted by arrows starting from the zone separation spray devices. Without zone separation sprays the cooling water on the top surface of the plate or strip tends to flow both upstream and downstream, in relation to the direction of movement of the plate or strip, of the apparatus for controlled cooling and consequently the cooling of the top surface will start earlier and/or finish later compared to the bottom surface. The cooling water sprayed onto the bottom surface does not flow upstream or downstream partly because the water naturally falls away under gravity and partly because it is blocked by the rollers 2 on which the plate or strip is being transported. The direction of movement of the plate or strip is depicted by an arrow at the right end of the plate or strip 1.
Plates with differing thicknesses and different metallurgical requirements have different cooling requirements and sometimes cooling over the full length of the apparatus for controlled cooling 9 is not required. In this case it is common practice to switch off some of the top and bottom headers so that a shorter length of the apparatus is used for cooling. For example in the simplified system illustrated in
For clarity
The principle of operation of the zone separation sprays is that the zone separation spray devices 10, 11 and 12 are arranged such that the jets of the zone separation sprays are angled in towards the apparatus for controlled cooling 9 and that they have sufficient impact pressure to prevent the cooling water from passing through the zone separation sprays so that the cooling water is forced to travel to the edge of the plate instead of flowing upstream or downstream.
The impact pressure is the product of the mass flow (kilogram per second, kg/s) times the speed (metres per second, m/s) divided by the area of impact (square metre, m2). The cooling flow of cooling water from the cooling headers on a modern apparatus for controlled cooling can be up to 33 liters per second per square metre or more at maximum flow. Consequently the zone separation sprays require a high mass flow and a high speed to achieve sufficient impact pressure to prevent that large amount of cooling water from passing through.
A zone separation spray device comprises one or multiple rows of nozzles. Multiple rows are often used so that the first row can stop most of the cooling water from passing through whilst subsequent rows stop any cooling water that gets through the first row.
A significant problem with a prior art apparatus for controlled cooling as illustrated in
When cooling is done with maximum cooling flow from the cooling headers the flow out of the zone separation spray devices is only a small proportion of the total flow, typically less than 10%.
If when working at low cooling flows the same large impact pressure and large flow out of the zone separation sprays that are used during maximum cooling flow are employed, the flow out of the zone separation spray devices is a much higher proportion of the total flow, typically more than 50%.
The cooling flow is the sum of cooling water flows applied by all the top cooling headers 6 in operation. The flow out of the zone separation spray devices is the sum of fluid flows applied by all the zone separation spray devices in operation. The total flow is the sum of the cooling flow and the flow out of the zone separation spray devices. The flow out of the zone separation spray devices contributes to the cooling.
In the above example of a turndown ration between maximum and minimum cooling flow the proportion of the flow of the zone separation sprays will be twenty times higher at minimum cooling flow than at maximum cooling flow. By that large amount of fluid applied by the zone separation sprays in addition to the cooling flow the cooling process is affected negatively, since top and bottom cooling may become unequal due to large differences between the amounts of fluids which cool the top surface and the underside.
It will be apparent that whilst the description above refers to a MULPIC type cooling system the principle applies to any cooling system which utilizes zone separation sprays to constrain the cooling water on the top surface of the plate or strip being cooled.
It is the object of the present invention to provide a method and an apparatus for controlled cooling of hot rolled plate or strip shaped metals which diminishes affect of the cooling process by the contribution of the flow out of the zone separation spray devices to the total flow.
This object is solved by the provision of a method for operating an apparatus for the controlled cooling of hot rolled plate or strip shaped metals by means of a cooling fluid comprising at least two zone separation spray devices for constraining the cooling fluid on the top surface of the plate or strip to an area, which is characterized in that during cooling the flow out of the zone separation spray devices is adjusted as a function of the cooling flow applied.
Cooling is done by application of a cooling fluid to the surfaces of the hot rolled plate or strip shaped metals. Preferably, the cooling fluid is water. The cooling flow is the sum of flows of cooling fluid applied to the top surface by means for cooling, for example top cooling headers as shown in
The zone separation spray devices apply fluid flows in the form of jets or streams on the plate or strip shaped metal to constrain the cooling fluid on the top surface of the plate or strip to a desired area. These fluid flows are called zone separation sprays. The fluid is usually but not necessarily the same as the cooling fluid. The flow out of the zone separation spray devices is not part of the cooling flow.
By adjusting the flow out of the zone separation spray devices as a function of the cooling flow applied it can be ensured that the proportion of the flow out of the zone separation spray devices is a small proportion of the total flow at high cooling flow operation as well as at low cooling flow operation. The higher or lower the cooling flow, the higher or lower the flow out of the zone separation spray devices is adjusted, thereby always simultaneously ensuring that the impact pressure of the zone separation sprays is high enough to prevent cooling fluid from passing through and that its proportion of the total flow is small enough to not affect the cooling process significantly. The proportion of the total flow may be up to 20%, preferably up to 10%.
Each of the zone separation spray devices can either be adjusted independently from any other zone separation spray device or alternatively there can be a common system for adjusting several or all of the zone separation spray devices together.
The adjustment of the flow out of the zone separation spray devices can be done by any means for adjusting the flow of a fluid. Preferably, it is done by adjusting an adjustable valve and/or by changing the speed of a variable speed pump.
To accurately adjust the flow out of the zone separation spray devices it is monitored, preferably by measuring its pressure.
Furthermore, the object of the invention is solved by provision of an apparatus for the controlled cooling of hot rolled plate or strip shaped metals by means of a cooling fluid comprising at least two zone separation spray devices for constraining the cooling fluid on the top surface of the plate or strip to an area, characterized in that it comprises at least one device for controlling the flow out of the zone separation spray devices as a function of the cooling flow applied.
There can be one device for controlling the flow out of several or all the zone separation spray devices together. Alternatively for each one of the zone separation spray devices there can be one device for controlling the flow in another embodiment of the invention. The device for controlling the flow out of the zone separation spray devices may comprise at least one adjustable valve, and/or at least one variable speed pump.
For controlling the flow out of the zone separation devices the device for controlling the flow out of the zone separation spray devices may comprise at least one pressure transducer for measuring the pressure of the flow and/or at least one flow meter for measuring the flow.
Schematic representations of the prior art and of exemplary embodiments of the invention are described in schematic
For simplicity
In the exemplary embodiment illustrated in
An alternative embodiment can use a flow-meter instead of the pressure transducer 15.
The pressure transducer 15 is not absolutely necessary and an alternative embodiment of the invention is illustrated in
Another alternative embodiment is illustrated in
Number | Date | Country | Kind |
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08251349.0 | Apr 2008 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP09/53248 | 3/19/2009 | WO | 00 | 1/7/2011 |