Method and device for enhanced strip cooling in the cold rolling mill

Abstract

 The present invention relates to a device (8) for cooling a flat or long metallurgical product (1) in a cold rolling mill, by creating a highly turbulent liquid coolant cushion at low pressure, comprising a header made of one or more compartments (9), each compartment (9) being provided with a means for supplying the liquid coolant (10) and with an internal surface to be located at short distance of the product (1), said internal surface comprising a plurality of nozzles or bore holes arranged according to a well-defined two-dimensional pattern (11).

Description

Field of the invention

[0001] The present invention relates to an enhanced metal strip cooling method, and particularly to the so-called water pillow cooling (WPC) technology, to be applied in the cold rolling mill.

[0002] The invention also relates to the device for carrying out the method.

Technological background and prior art

[0003] In order to reduce the cost ratio in cold rolling mills, managers are continuously increasing the productivity thereof. Combined with the trend to roll strips with increased rolling speed, higher reduction, higher strength and with a very high surface quality requirement, the mills are confronted with several operational problems.

[0004] One of the main restrictions is the increase of heat generation in the roll bite, which transfers further load onto the mill cooling system in order to control roll and strip temperatures and leads to a decrease of lubrication properties. It is known that, due to this higher roll bite temperature, the lubricant will lose its viscosity and this will at a certain point lead to a breakdown of the lubrication film.

[0005] As current mill cooling systems are not well adapted, this leads to either strip defects generation that affect product quality (e.g. heat scratches or streaks, heat and shatter marks, flatness defects) or to a decrease in rolling speed (necessary to avoid these defects, and to avoid quickly rising rolling forces).

[0006] Therefore improved cooling is today a key requirement to increase mill performance and to assure the competitiveness of the cold rolling mill. Particularly, it becomes more and more important to control the roll bite temperature at higher cold rolling speeds by enhanced homogeneous cooling strategies such as the water pillow cooling (WPC) technology. The principle of WPC, a well-known technology developed by the Applicant as from 1984, consists in building up high turbulence in a water cushion created on the surface of the product to be cooled thanks to the injection of straight water jets through nozzles or drilled holes in a header.

[0007] Moreover, a related cooling technology, the so-called high turbulence roll cooling (HTRC), was already developed and patented by the Applicant under WO 2008/104037 (US 2010/0089112, etc.). It is related to a highly turbulent roll cooling device working at low pressure, under the form of a concave header provided on its internal surface with a determined pattern of nozzles or bore holes.

[0008] To date, intensive strip cooling, based on the turbulent WPC cooling principle is only applied in hot rolling generating a high heat flux. While strip cooling is extensively used in various hot rolling conditions in order to obtain well-defined metallurgical properties (e.g. harder grade steels), up to now intensive strip cooling was not considered critical in cold rolling operations. In many cases, strip cooling only consists in applying roll coolant further flowing on the strip or in using an additional row of nozzles spraying on the strip. The necessity to improve cooling performance in order to suppress temperature related defects is however recognized.

[0009] As mentioned above, there are a few surface defects in cold rolling that can directly be related to high roll and/or strip temperature. More precisely, by far the most well-known is heat scratch. Other defects are related to the application of the coolant. An example of these defects is chatter, which can be due to coolant disturbance of the oil film layer and thereby of the coefficient of friction in the roll bite. Heat scratches (also called heat streaks) are surface defects that often occur in cold rolling with high reductions, so it is often found in tinplate rolling. Heat scratches are formed when the lubrication film breaks down.

Aims of the invention

[0010] The present invention aims at providing an efficient strip cooling at the cold rolling mill which, associated to efficient roll cooling, results in an increased rolling speed together with a decreased occurrence of temperature related heat defects.

[0011] The invention also aims at obtaining a lower temperature in the roll bite to obtain enhanced lubrication properties (or higher viscosity) and reduced scattering defects.

[0012] The invention also aims, thanks to using an efficient low pressure cooling technology developed by the Applicant, at reducing electric pump energy and further at contributing to lower environmental impact.

Summary of the invention

[0013] A first aspect of the present invention is related to a device for cooling a flat or long metallurgical product in a cold rolling mill, by creating a highly turbulent liquid coolant cushion at low pressure, comprising a header made of one or more compartments, each compartment being provided with a means for supplying the liquid coolant and with an internal surface to be located at a short distance of the product, said internal surface comprising a plurality of nozzles or bore holes arranged according to a well-defined two-dimensional pattern.

[0014] According to preferred embodiments, the device for cooling a flat or long metallurgical product in a cold rolling mill is further limited by one or a suitable combination of the following characteristics:

• the nozzles are interchangeable and have a diameter comprised between 1 and 5 mm ;
• the liquid coolant is an emulsion ;
• the emulsion consists essentially of water comprising from 0 to 1 % of oil ;
• the internal surface of the header is designed so as to match a strip surface, and a plurality of compartments are disposed along a direction of movement of the strip.

[0015] A second aspect of the present invention relates to a cold rolling mill, comprising at least a rolling stand with a pair of work rolls and a product cooling device according to the invention, wherein the product cooling device is located at the entry or exit side of the work rolls.

[0016] According to preferred embodiments, the cold rolling mill is further limited by one or a suitable combination of the following characteristics:

• the product cooling device is located between two stands thereof ;
• the product cooling device is located only on one or on both side(s) of the product ;
• the product cooling device is integrated inside a guidance table of the mill that is able to rotate from a horizontal position to a vertical position, so that work rolls are built out and vice versa.

[0017] A third aspect of the invention relates to a method for cooling a flat or long metallurgical product in a cold rolling mill, by using the cooling device of the invention, comprising the following steps:

• positioning said cooling device on the entry or exit side of work rolls of a rolling stand or between two rolling stands, and on the top and/or bottom side of the product, so as to create a gap between the internal surface provided with nozzles and the product being comprised between 5 and 200 mm ;
• supplying one or more compartments of the header of the cooling device with a liquid coolant and spraying the latter into said gap on the product ;
• adjusting the pressure of the liquid coolant to a value of between 0.1 and 4 bar and the specific flow rate between 10 and 500 m³/hour/m², in order to create in said gap a liquid coolant cushion in a highly turbulent state.

[0018] Preferably, the gap between the cooling device and the product, the pressure of the liquid coolant and the specific flow rate are adjusted so that the temperature of the product on the exit side of the work rolls of the rolling stands does not exceed 170°C.

Brief description of the drawings

[0019] FIG. 1 schematically represents the principle of WPC strip cooling applied to the cold rolling mill.

[0020] FIG. 2A schematically represents a design of WPC strip cooling header according to the present invention. FIG. 2B shows a static test using a transparent strip.

[0021] FIG. 3A schematically represents flow patterns versus specific flow rates. FIG. 3B represents the flow rates obtained with the WPC strip prototype header (lower view).

[0022] FIG. 4 is an example of schematic layout for the experimental setup according to the invention.

[0023] FIG. 5A represents the impact of the WPC strip cooling on the rolling force and the strip temperature out and FIG. 5B represents the corresponding influence on the strip surface quality (lower views).

[0024] FIG. 6 represents an example of impact of cooling flow rate on the exit thickness.

[0025] FIG. 7 is a compared calculation of the friction coefficient with and without strip cooling.

Description of preferred embodiments of the invention

[0026] Figure 1 schematically shows the implementation of a WPC strip cooling system at the cold rolling mill according to the present invention. Two stands (n, n+1) have been represented on FIG. 1. Each stand has two work rolls 2, and on its strip entry side, a strip lubrication system 5 provided with neat oil and a cooling and lubrication system by emulsion 6. At the exit side of stand n is a roll cooling emulsion system 7, a wiping roll 3 and a tension roll 4.The WPC strip cooling device 8, according to an embodiment of the present invention, is also located at the exit side of the first stand n.

[0027] The cooling system should enable higher rolling speeds without running into temperature related strip defects. Inevitably the necessary pump energy should decrease because the turbulent cooling techniques operate at low pressure. This has been simulated in the experimental tests described below to determine the optimum parameters.

[0028] In strip cooling, the water cushion is determined by the coolant flow and the evacuation of the coolant liquid. This clearly indicates that each application requires a detailed study of coolant flow and coolant performance strongly influenced by the size and geometry of the cooling headers. Thus, the scope of the present invention is not restricted by the detailed engineering characteristics presented below.

Laboratory characterisation of the WPC strip cooling prototype

[0029] The Applicant has constructed a WPC strip cooling prototype (FIG. 2A) that has been tested in a cold rolling pilot line. This prototype header 8 is designed with similar dimensions as it would be implemented in a corresponding industrial cold rolling mill (but only with a much smaller width). The strip cooling unit 8 that was designed for the pilot line experiments is internally divided in three chambers 9. Each chamber 9 can be "switched on" or "switched off" (i.e. fed or not with coolant) during use, thanks to its main feeding pipe 10. The surface of the header 8 facing the strip 1 is provided by a specific pattern of nozzles 11. These nozzles 11 that are used in the unit are designed to be easily changed, according to different sizes. For example, nozzle diameters of 2, 3 and 4 mm were used during the test. The flow rate control was achieved with a pump-unit, at a maximum flow rate of 350 l/min. Offline tests were performed on the Applicant cooling simulation platform, which comprises a static transparent plate 18 (FIG. 2B).

[0030] In addition, the cooling pattern of the different cooling sprays was monitored with their specific flow (FIG. 3A) to see the WPC coverage pattern. Different nozzles sizes and pressures were selected in order to meet the industrial available flow rates (FIG. 3B). According to the specific flow and experience from other WPC implementations, it was stated that a sufficient cooling should be reached during the cold rolling pilot line tests performed on the industrial multi-mill.

Experimental procedures

[0031] Cold rolling pilot line tests were performed at an industrial cold strip mill with a WPC strip cooling unit prototype. The strip cooling unit, pyrometers and a lubrication system were integrated in a cold rolling pilot facility. The full layout is schematically given in FIG. 4.

[0032] The mill was used in the two-high configuration with a work roll 2 diameter of 397 mm and a Ra-roughness of 1 micron. The material grade (temper 61 C) had an initial strip thickness of 1.82 mm. The material width was 100 mm. It is a single stand reversible mill (uncoiler 12, coiler 13) that can run in two-high (work roll 2 diameter = 400 mm) or four-high configuration (work roll 2 diameter = 140 mm). An entry roll cooling 16 is provided to the work rolls 2. Temperature at the entry side and the exit side of the work rolls 2 are measured by pyrometers 15. All relevant process parameters are automatically logged. The specifications of the mill enable the simulation of rolling process in production mills. Coolant emulsion is applied by a recirculation system (capacity of 4000 I). Extra lubricant was applied with Direct-Application unit (DA) 14.

[0033] During the tests, the second pass was under focus since this pass is especially known to be critical for heat scratches. The first pass was rolled without lubrication and entry/exit cooling in order to put some temperature in the strip. The only lubrication was obtained from the oil on the strip from the pickling line. The thickness of the strip after first pass was 1 mm. The coil was then rewound. The second pass was rolled with DA-lubrication and entry roll cooling was used in order to simulate real rolling conditions. Reductions of the second pass varied between 45% and 60%. Entry and exit tension-force were typically 20 and 12 kN, respectively.

[0034] From the tests, it was found that the strip cooling has an enormous influence on the process parameters. This is especially true when the temperature in the roll bite becomes high, due to high reduction or high incoming strip temperature from the previous stand. The most obvious observation is given in figure 5A (upper view) where the roll force drops by over 30% and the rolling power by 10% only by using the strip cooling. The exit strip temperature dropped with about 20°C from 190°C to 170°C (figure 5A, lower view). Decreasing the flow from 20 m³/h to 10 m³/h had a minor influence on the process, which means that even at 10 m³/h there was an efficient cooling.

[0035] Furthermore, the desired reduction was 55% which could only be achieved with a strip cooling. The influence of the strip cooling on the appearance of scratches has been shown convincingly several times during the test. An example of heat scratches 17 is shown in figure 5B. The first strip, that was rolled at a reduction of 55%, without strip cooling, clearly shows heat scratches (see upper view). During rolling, when the strip cooling was switched on, the scratches disappeared (see lower view). It should be mentioned that no other rolling parameters were changed.

[0036] Not all experiments could be used for this comparison as in some cases the desired reduction could not be obtained without strip cooling. In such a case the decrease in force/power was very low, but more reduction could be made. Such an example is given in FIG. 6.

[0037] The necessity to cool the strip surface has been clearly identified, when the strip temperature increases to a critical temperature of about 200°C. The difference between the final strip temperature with and without strip cooling is roughly 12°C. The effect on the process parameters and surface appearance depends on the final strip temperature: small influence if temperature is relatively low, and can be very highly influenced if the strip temperature is high which means oil that is deteriorating. By using the strip cooling unit, the rolling force decreased by 5 to 30% and the rolling power by 4 to 10%. The higher the temperature in the roll bite, the higher the decrease in force/power. An additional test was done to compare conventional flat spray nozzle strip cooling (7 bar) with the WPC technology (below 2 bar). It was observed that WPC had the same cooling performances with a much lower pressure, so energy consumption. Also, a thermal asymmetry on transverse strip temperature profile by conventional non-homogeneous cooling techniques (e.g. nozzle overlap) is frequently observed at mill exit and tends to degrade the strip flatness. A direct result of an efficient strip and roll cooling is an increased rolling speed and a decreased occurrence of temperature related heat defects. This is especially the case for cold rolling mills producing at the limits of their capabilities. It can be concluded that the higher strip cooling efficiency will lead to better cold rolling process.

Impact on the friction coefficient

[0038] The data of the cold rolling pilot line tests with strip cooling were used for extra parameter calculations. For all experiments in which the second pass was simulated, the coefficient of friction (COF) was calculated with a finite difference model. The result is shown in FIG. 7, in which a distinction is made between the processes where the strip cooling unit was either used or not.

[0039] In FIG. 7, it can be clearly seen that below 170°C, the coefficient of friction is rather constant. In fact, below 170°C, the coefficient of friction slightly increases with strip temperature as the viscosity of the oil decreases with increasing temperature. Above 170°C, the coefficient of friction starts to increase very much (roughly 20% increase for an increase in strip temperature of 30°C).

[0040] Apparently, the oil loses very quickly its lubricating properties above this critical temperature. The scratches that were observed during some of the experiments were also made when the strip exit temperature was above 170°C. It is remarkable that the COF as a function of the measured strip temperature has a better defined line for the experiments where the strip cooling unit was used. Probably, this is related to the fact that using the strip cooling unit gives a very well defined entry strip temperature. If the strip cooling unit is not used, the strip entry temperature depends on the time between pass 1 and 2 which is obviously variable in an experimental surrounding.

Conclusion

[0041] Cold rolling pilot line tests were performed at an industrial cold strip mill with a WPC strip cooling unit prototype. Problems that occur in industrial rolling mills, such as scratches and very high rolling force, have all been simulated on the pilot mill. These phenomena could be induced by making a critical temperature of about 200°C in the roll bite (by increasing the reduction). The benefits of a WPC strip cooling device have been clearly demonstrated in this pilot mill series. The undesirable phenomena disappeared when the strip cooling was used. By using the strip cooling unit the rolling force decreased by 5 to 30% and the rolling power by 4 to 10%. The higher the temperature in the roll bite, the higher the decrease in force/power. The reduction of temperature at the roll bite results in better lubrication properties and further to higher rolling speeds and higher production rates. It is also possible that the oil present in the emulsion increases the plate-out of the strip surface due to the turbulence of the emulsion at the strip surface. Furthermore, thanks to the efficient low pressure cooling technology developed by the Applicant, the reduction of electric pump energy by using WPC contributes to a lower environmental impact.

Symbols of reference

[0042] 

1. Metal strip

2. Work roll

3. Wiping roll

4. Tension roll

5. Strip lubrication (neat oil)

6. Roll cooling & lubrication by emulsion

7. Roll cooling by emulsion

8. Strip cooling header (WPC)

9. Header chamber

10. Main feeding duct

11. Cooling nozzle pattern

12. Uncoiler

13. Coiler

14. Direct oil application (DA)

15. Pyrometer

16. Entry roll cooling

17. Heat scratches

18. Transparent test plate

Claims

1. A device (8) for cooling a flat or long metallurgical product (1) in a cold rolling mill, by creating a highly turbulent liquid coolant cushion at low pressure, comprising a header made of one or more compartments (9), each compartment (9) being provided with a means for supplying the liquid coolant (10) and with an internal surface to be located at short distance of the product (1), said internal surface comprising a plurality of nozzles or bore holes arranged according to a well-defined two-dimensional pattern (11).

2. The device according to Claim 1, characterized in that the nozzles are interchangeable and have a diameter comprised between 1 and 5 mm.

3. The device according to Claim 1, characterized in that the liquid coolant is an emulsion.

4. The device according to Claim 3, characterized in that the emulsion consists essentially of water comprising from 0 to 1% of oil.

5. The device according to Claim 1, characterized in that the internal surface of the header is designed so as to match a strip surface, and a plurality of compartments are disposed along a direction of movement of the strip.

6. A cold rolling mill, comprising at least a rolling stand with a pair of work rolls (2) and a product cooling device (8) according to Claim 1, characterized in that the product cooling device (8) is located at the entry or exit side of the work rolls (2).

7. The cold rolling mill according to Claim 6, characterized in that the product cooling device (8) is located between two stands thereof.

8. The cold rolling mill according to Claim 6, characterized in that the product cooling device (8) is located only on one or on both side(s) of the product.

9. The cold rolling mill according to Claim 6, characterized in that the product cooling device (8) is integrated inside a guidance table of the mill that is able to rotate from a horizontal position to a vertical position, so that work rolls (2) are built out and vice versa.

10. A method for cooling a flat or long metallurgical product (1) in a cold rolling mill, by using the cooling device (8) of Claim 1, comprising the following steps:

- positioning said cooling device (8) on the entry or exit side of work rolls (2) of a rolling stand or between two rolling stands, and on the top and/or bottom side of the product, so as to create a gap between the internal surface provided with nozzles (11) and the product (1) being comprised between 5 and 200 mm ;

- supplying one or more compartments of the header of the cooling device (8) with a liquid coolant and spraying the latter into said gap on the product (1) ;

- adjusting the pressure of the liquid coolant to a value of between 0.1 and 4 bar and the specific flow rate between 10 and 500 m³/hour/m², in order to create in said gap a liquid coolant cushion in a highly turbulent state.

11. The method according to Claim 12, wherein the gap between the cooling device (8) and the product (1), the pressure of the liquid coolant and the specific flow rate are adjusted so that the temperature of the product on the exit side of the work rolls (2) of the rolling stands does not exceed 170°C.

Drawing