Patent Application
20200188975
The invention relates to the field of rolling mills, and in particular reducing metallurgical property transition in inline heat treatment for long rolling products.
Prior to 1980 the system had a very crude temperature control which relied heavily on operators to make adjustments when temperature set point changes where required. The pressures where then fixed for the rolling process and did not adapt to the change in process conditions (furnace exit temperature, change in product dimension, water temperature etc.). The current system, in use since the mid-1980s, uses an operator defined recipe system and is able to automate the cooling process by obtaining information from the zone finishing pyrometer and provide small incremental changes to the modulating valve as processing conditions change over time.
A typical mill has fix speed centrifugal water pumps at the water treatment facility. These pumps are often very large and provide the total flow for all of the equipment of the entire rod/bar mill at a relatively low pressure (˜2 bar). Often the mill reheat furnace is on its own supply, since any reduction in the required pressure/flow could be catastrophic to this area. The large pump is often followed by a series of smaller booster pumps to increase the pressure depending on equipment design. Typical pressures are 6.0, 8.0, 10.5 and 14.0 bar: The 6.0 bar is used for rod mill water box, roughing mill roll cooling, and most other equipment. 8.0 bar is used for the high speed rolling equipment that uses tungsten carbide rolls and for processing rod quenched and tempered rebar. 10.5 bar is used for cooling bar mill plain round products using the bar mill water boxes. Finally 14.0 bar is used for quenched and tempered bar mill rebar product. The pressures are oversized to account for pressure losses in piping and valves used to achieve target processing conditions.
A ‘Cooling Zone’ is a section of the mill to cool the product to a desired metallurgical set point before additional processing or finishing equipment. For each zone there is an exit pyrometer in which the operator defines the set point temperature. Standard rod mill cooling model has three (3) cooling zones. One before the No-Twist Mill (PreNTM), one after the No-Twist Mill (PostNTM) and one after the Reducing Sizing Mill (PostRSM). Each zone has a Pressure Reducing Valve (PRV) to help balance the required supply pressure for the zone. The zone pressure target is determined by the operator to achieve water box set point targets within the operating range of the valves (20-80% of full open position). A pressure transmitter, located directly after the PRV, sends a signal to the PLC where a ‘Pressure Loop’ program informs the valve to adjust and maintain the desired pressure.
Each zone typically consists of one (1) to four (4) water boxes. Each rod mill waterbox is either 4.7 or 6.1 m long. The quantity and length of the water boxes are defined by the mills worst case processing conditions and the amount of quench time and equalization (relaxation) required to achieve the metallurgical defined set points. Design variables include: mill tonnage rate, max mill speed, grade set point temperature, furnace exit temperature, and number of stands. The worst case is a single finish dimension being processed at the fastest tonnage rate with the longest cooling requirement. All other cases require less water contact time. This equates to less cooling length since each product speed is roughly fixed. The rod mill cooling nozzle typical design pressure is 2.0 bar.
When the water pressure is less than a minimum pressure (˜0.5 bar); the product cooling is not effective in penetrating the steam jacket (created by the Leidenfrost effect) and cooling the product. When the pressure is too high (˜3.0 bar) the product surface is over-quenched and an undesirable allotrope of steel called martensite can form. In the case of the quenched and tempered process in rod mills, a minimum water pressure of 5.0 bar is the design parameter to create martensite on the surface of the finished rod product. Bar mill cooling nozzles are of a different design and require a high pressure to overcome its significant pressure loss. The bar mill set point pressures for plain carbon and quenched and tempered are 7.0 and 12.0 bar respectively.
Since the finished product is made from billets, the batch process is consistently rethreaded for each billet. The front end, and in some cases tail end, must be hot to prevent the product from cobbling and achieve proper laying pattern onto the cooling conveyor correctly. This threading length is called the ‘hot length’ since it is roughly the same temperature as the furnace exit billet temperature. To keep the ‘hot length’ at elevated temperatures, the water boxes are turned off as the head passes though. The time between the finish of the previous product and the start of the next (billet gap) can be as short as 2 seconds. In order to divert the water flow away from the product quickly, a 3-Way Divert Valve is used. After the hot length passes, the 3-Way Divert Valve is then shifted quickly to redirect water to the product.
The ‘hot length’ distance is an operator input value in the ETCS and is determined by: product cross sectional dimension, and distances between controlling equipment. Once the water is directed to the product, a period of time occurs for the water to reach the set point pressure/flow. This period of time is called the ‘transition length’ where final metallurgical properties have not yet been achieved.
The transition length is cooler than the ‘hot length’ and typically looks the same temperature as the set point temperature of the finished product. The predominate method to determine the transition length is to cut several tensile strength samples and record the tensile strength as it reaches the set point value. Transition of scale thickness can also be an indicator of where the transition length begins and ends. Depending on end user's quality expectations; most mills must trim the ‘hot’ and ‘transition length’ to give the end user customer the desired product
According to one aspect of the invention, there is provided a temperature control system used for cooling a rolling mill product. The temperature control system includes a plurality of isolation valves that are directly coupled to one or more water boxes. A pump is coupled to the isolation valves. The pump provides the pressure needed for cooling. The isolation valves are positioned to reduce the time required to build up pressure for cooling and reducing the metallurgical property transition length of the rolling mill product.
According to another aspect of the invention, there is provided a method of configuring a temperature control system used for cooling a rolling mill product. The method includes positioning a plurality of isolation valves that are directly coupled to one or more water boxes to reduce the time required to build up pressure for cooling and reducing the metallurgical property transition length of the rolling mill product. Also the method includes coupling a pump to the isolation valves to provide the pressure needed for cooling.
The invention involves a system that increases front end uniformity of metallurgical properties and scale control for inline heat treatment of long rolling products by moving isolation valves closer to the cooling operation. Isolation valves are mounted to the waterbox or within close proximity of the cooling nozzle. Each isolation valve is associated with a single cooling nozzle. This offers a more discrete control of cooling length which will increase cooling efficiency. The isolation valve is able to open and close full stroke in less than a second which minimizes the time required for the cooling nozzle to achieve set point pressure. A pressure reducing valve may be incorporated to alleviate supply pressure differentials due to operation of other cooling zones or equipment. A variable frequency drive pump is able to adjust the supply pressure to meet the set point temperature in order to achieve target temperature for single and/or multiple zones.
The isolation valve 4 response, from fully closed to fully open (˜290 ms), matches closely to the guaranteed of the modified Fisher 3-Way Divert Valve (˜300 ms) used in the prior art. This is possible since the size of the 2″ isolation valve is significantly smaller than the 4 to 8″ Fisher 3-Way Divert Valve. Since the isolation valve's 4 maximum dimension is 2″; it can only effectively process water suitable enough for one cooling nozzle. This means that there can only be one (1) secondary valve for each cooling nozzle. This greatly increases process control by giving the ability to uniformly predict how much pressure/flow is changed when a valve is actuated.
When the isolation valve 4 is closed the water pressure will be slightly higher than the required cooling set point. This increase in water pressure will change depending on the set point pressure. It has been observed that for 2.0 bar set point; the increased change in pressure is ˜0.5 bar.
Since the water is no longer being diverted, the process will now stop the flow (‘dead head’) rather than divert the flow. When the downstream valves offer more resistance; the pressure after the pump will increase. Using a fixed speed pump, the pressure can only be increased by a factory determined set tolerance before the shut-off pressure is reached, as shown in
With a Variable Frequency Drive (VFD) pump 8, the process can continue to run for short periods of time while the system is dead headed to protect the water pumps from electrical/mechanical overload. Utilizing a VFD pump 8 reduces waste by only providing the pressure required; rather than having a fixed speed booster pump designed only for the worst case scenario water pressures and flows. The VFD 8 pumps provide the rolling mill with better energy efficiency and reduced water consumption that requires filtration and cooling. The VFD 8 pumps also provide better set point pressure stability. An Armstrong VFD Pump, as shown in
For typical rod mill systems: only a single VFD pump per strand is needed. Having separate supply pumps allows each strand to run independent of each other's processing conditions. Installing a standby pump is recommended to maintain production capability.
By removing the standard 3-Way Divert Valve and utilizing the VFD pump 8; the Back Pressure Valve (BPV) and its control loop is no longer required. This reduces the complexity of the system by delivering the required set point pressure without additional controls.
By treating an entire zone 1, 2, and 3 as a single water box, only one pressure set point (initial set point 2.0 bar) is needed. This eliminates the requirement for modulating valves at each water box and their corresponding flow transmitters and flow loops. The overall recipe and ETCS system will only require: number of cooling nozzles per zone, zone pressure set point (with corresponding position of the zone pressure reducing valve (PRV) 12), and the strand's pressure set point (with corresponding torque of VFD Pump 8).
The zone PRVs 12 can modulate the pressure as rolling condition parameters change based on zone entry and finishing temperatures. The zone entry temperature can help predict what is required while the zone exit pyrometer can fine tune the required finish temperature. Since the process needs to maintain the pressure between a range (0.5 and 3.0 bar for rod mills) a pressure transmitter is needed for each zone. In the event the water pressure falls below the required minimum for the zone the isolation valve will close to consolidate the cooling toward the finishing end of the zone.
The zone PRVs 12 are used to set the water pressure for the zones 1, 2, and 3. Each zone 1, 2, or 3 is able to have different set points depending on processing conditions. It is critical that the supply pressure for each zone is maintained so that the process has the correct pressure set points for each zone.
The pressure transmitters 10 offer <1 millisecond response time which is much faster than the 145 millisecond response rate of the Rosemount 2088. However, the accuracy of the pressure 10 is <+/−0.17 bar typical (+/−0.34 bar max) a comparison to the standard Rosemount 2088 (+/−0.14 bar) transmitter must be further evaluated. The Rosemount 2088 transmitter requires an expensive HART 475 communicator to set the device. The accuracy should be sufficient enough for the process. By focusing the modulation control to be based on temperature with a pressure reference the overall cost of the system can be drastically reduced. The expensive flowmeter with HART 475 communicator is no longer required.
Several types of valves were tested in order to find the best fit for the invention.
Fisher 3-Way Divert Valve claims to have 0.300 second actuation time. Through testing it was found that the valve does actuate within the 0.300 seconds however; the response rate to achieve steady state pressure is significantly longer than initially expected. Due to the distance of the pipe length (7.7 m) and increased resistance of altitude (2.3 m) the response rate is delayed significantly. In the test setup 1.3 seconds was required to achieve design pressure (2.0 bar).
Ross Dale CX valve claimed to have 0.014 second response rate. Although some testing proved this to be true the valve can only operate if the pilot air supply is higher than the working pressure of the medium. Therefore for 2.0 bar water operating conditions the typical design of 4.1 bar of air is sufficient. However, as the conditions change and higher water pressures are required, the 4.1 bar air pressure is not strong enough to consistently close the valve. After cycle testing, half year (˜250,000) cycles, the response rate was diminished and the seals needed to be replaced. In
This valve is an industrial valve used in paper mill applications to process thick slurries. It has a piston, 90 mm operator, which creates a positive seal when closed. The valve is available in fail open or fail closed default positions. The valve requires an externally mounted actuator; the ASCO 8317 was selected for its fast response rate and high flow coefficient value. The actuator is operated by a 24 Volt DC electrical supply and 4.1 bar instrument air supply, which is typical of rolling mill conditions. The valve data sheet claims to have 0.290 second response rate. Throughout testing this appeared to be consistently true even after cycle 500,000 cycles (1 year of typical production). Technical professionals at ASCO have claimed that this valve should be able to operate 1 million cycles without maintenance. The standard design of 4.1 bar of pilot air pressure is sufficient to close the valve to the guaranteed 6.2 bar incoming water pressure. Higher operating water pressures of 8.0 bar have been observed and the valve continues to perform as designed. In
With a fixed speed pump, as the downstream control valve begins to close to decrease product cooling; the water pressure begins to increase as the total water requirement is reduced. In
The VFD Pump 8 is used to control torque when flow change is required while maintaining stable supply pressure. Pump selection discussed in this report only account for variable head. When the static head speed is reduced to a point where the flow is 0 LPM; the system pressure is equal to static head pressure. The static head pressure will vary depending on piping and elevation.
The VFD Pump 8 selected allows the process to deadhead for short period of time (<30 seconds) without increasing pressure. When the system must deadhead for more than 30 seconds the pump can shut off. In
By installing a pressure transmitter 10 downstream of the pump 8 and relaying information back to the drive, constant pressure can be maintained while the flow is decreasing. Through extensive testing of flow characteristics of the cooling nozzles the pressure vs flow relationship for various bore sizes has been defined. Using this data as the pump system curve; electrical and automation can map the required speed (or torque) to meet the E&A process set point.
In order to optimize pump selection, the Best Efficiency Point (BEP) is used. The BEP is a point on the pump curve where the efficiency is the highest. At this point, the impeller is subjected to minimum radial force promoting a smooth operation with low vibration and noise. When the flow conditions change the efficiency is degraded for that pump. Armstrong double volute pump with VFD is selected increase process capabilities as flow requirements change.
The invention improves acceleration of water pressure to reach steady state conditions to minimize transition length and achieve desired metallurgical properties of the final product. First, the invention replaces the large and expensive 3-Way Divert Valves with several smaller isolation valves close to the rolling process. By strategically placing the isolation valves closer to the cooling process, the lag time to achieve desired set point temperatures has been decreased significantly. Furthermore, the supply pumps must be variable frequency driven in order to accommodate the ‘dead heading’ and maintain pressure set points with increased accuracy.
Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.
This application claims priority from provisional application Ser. No. 62/778,486 filed Dec. 12, 2018, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62778486 | Dec 2018 | US |
