Claims:WE CLAIM:

1. A system for simulation of accelerated cooling of hot rolled steel on ROT of hot rolling mill comprising of:

water pumping unit for supplying water to two different types of cooling arrangements for extracting heat from top and bottom surfaces of hot rolled steel;

top laminar cooling unit for application of cooling water on top surface of the steel;

bottom spray jet cooling unit for application of cooling water on bottom surface of the steel;

control unit for varying flow rate and pressure of water in top and bottom cooling units; and

water recirculation arrangement for conservation of cooling water.

2. The system for simulation as claimed in claim 1, wherein the said water pumping unit comprises of:
a concrete water sump for storing water (typical capacity 3-4 m3);

a water pump set for discharge water to cooling units (typically of 150-200 m3/hr at max. 20 m head);

a set of strainer and filter for removing solid contaminants from the cooling water;

and

connecting pipelines for supplying water to cooling units.

3. The system for simulation as claimed in claim 1, wherein the top laminar cooling unit further includes:
an overhead steel water tank (of 2-3 m3 capacity) placed at a height such that the water fed to the tank from the pumping unit is maintained at a constant head suitable for achieving laminar flow when discharged through headers; and

a series of headers (typically 10 No.) fitted with delivery tubes (around 20 no’s per header).

4. The system for simulation as claimed in claim 1, wherein the said bottom spray jet cooling unit comprises of:

a series of spray headers (typically 10 No.), each fitted with nozzles (7-8 No.) and placed between rollers of the ROT such that the spray jet of water impinges on bottom surface of the rolled steel over typical cooling area of 2000 mm long and 300 wide suiting to space available in the ROT; and

a pressure relief valve fitted in the delivery pipeline of the water pump along with a bypass or return line to control the spray pressure in the typical range of 0.5-
1.5 bar.

5. The system for simulation as claimed in claim 1, wherein the said control unit comprises of:

motorized valve for flow control in top and bottom cooling units;

manual valves for selection / de-selection of individual header for cooling purpose;

pressure relief valve for control of flow pressure;

selection switches fitted in the control desk of mill for operation of cooling system with provision for linking to PLC based control system of the mill;

pressure gauges for measurement of water pressure;

level indicator in the overhead tank for showing water level; and overflow line in the overhead tank for maintaining constant water head.

6. The system for simulation as claimed in claim 1, wherein the said recirculation of water for conservation purpose further comprises of a water collecting tray at bottom of the ROT with suitable pipeline for taking the used water back to the concrete water sump.

7. The system for simulation as claimed in claim 2, wherein the water pump set is configured for a flow rate of 150-200 m3/hr at the maximum water head 20 m.

8. The system for simulation as claimed in claim 2, wherein the strainer and filter is configured for filtering contaminants of size 100-200 microns.

9. The system for simulation as claimed in claim 3, wherein the series of headers are fitted with gooseneck delivery tubes for achieving laminar flow of water over typical cooling area of 2000 mm long and 300 wide suiting to space available in the ROT.

10. The system for simulation as claimed in claim 1, is suitable for

studying the effect of varying cooling rates (typically from 5-30o C/s) on properties of hot rolled steel in combination with different heating and rolling regimes;

validation of mathematical heat transfer and microstructure models developed for

ROT cooling of steel;

studying the effect of heat transfer additives on cooling rate of steel;

developing structure-property correlation for conventional and new steels;

studying the effect of differential cooling rate on shape or internal stress of the rolled steel.

Dated: this 13th day of February, 2017.
(N. K. Gupta)
Patent Agent
Of NICHE
For SAIL

To,
The Controller of Patents,
The Patent Office, Kolkata.
, Description:SYSTEM FOR PHYSICAL SIMULATION OF ACCELERATED COOLING IN HOT ROLLING MILLS

FIELD OF THE INVENTION:

The present invention relates to a system designed for physical simulation of forced cooling of steel immediately after hot rolling at any rolling mill such as hot strip mill and plate mill in order to achieve different properties of steel by varying cooling rates at the run-out-table (ROT). The said system can be mounted at the ROT of a laboratory rolling mill to study the effect of varying cooling parameters on mechanical and metallurgical properties of the steel in combination with different heating and rolling regimes.

BACKGROUND ART:

Hot rolling of steel is carried out with twin objectives of achieving desired dimensional qualities and mechanical properties of the rolled products such as strip, plate, bar, etc. The operation involves heating of the steel to a set temperature in the range of 1100-
1300 oC and then deforming it in hot condition at series of mill stands to achieve the

desired thickness and width. During heating and deformation of steel metallurgical changes take place in the steel depending upon its chemistry, temperature, strain and strain rate. Soon after the rolling, the steel is subjected to cooling operation for further metallurgical changes in the steel leading to the desired mechanical properties such as yield strength, ultimate tensile strength, ductility, hardness, fracture strength, etc. The cooling operation may involve simple natural air cooling or forced cooling with water. Most of the hot rolling mills adopt forced cooling using water as it leads to higher strength in steel compared to natural air cooling. For example, in plate mill it is called accelerated cooling and in hot strip mill it is called ROT cooling.

Properties of the rolled steel depend on its cooling rate, which in turn is dependent on flow rate, pressure and residence time of the cooling water over the steel. The cooling system may therefore have different types of arrangements for spraying water over the hot rolled steels. Some of the typical cooling arrangements are nozzle spray cooling, laminar cooling, water curtain, etc. With each type of the cooling system there is

provision of varying cooling rate by selecting different number and sequence of cooling headers or banks.

Now, the greatest difficulty in any industrial setup is that there is no scope of much experimentation with regard to varying cooling rate for achieving different properties of the finished steel. The problem further increases when new cooling rates are to be achieved either for different heating and rolling regimes or for production of the new steels. It is therefore highly required to have a laboratory facility for simulation of heating, rolling and cooling processes in tandem. Presently some of the steel producers or research laboratories are having experimental rolling mill to simulate reheating and hot rolling processes. One such experimental rolling mill (ERM) is also available with the organization of the inventors of the present invention. This mill was however not provided with any facility for simulating the said cooling process. Presently it was having a practice of either natural air cooling or water quenching of the hot rolled steel. The limitations of these two modes of cooling are that while in air cooling the cooling rate is very low, in water quenching it is extremely high and without any proper control. It was therefore required to develop an experimental cooling system for having provision of varying cooling rate with proper control. In order to have a proper simulation, the experimental system was also required to have cooling modes similar to its industrial counterpart. With this background the present innovated system was designed, fabricated, installed and evaluated.

OBJECT OF THE INVENTION:

The basic object of the present invention is to develop a system for simulation of accelerated cooling which would be adapted for implementing at the ROT of a laboratory rolling mill to carryout physical simulation of cooling process of industrial hot rolling mills.

Another object of the present invention is to develop a system for simulation of cooling which would be adapted to study the effect of varying cooling rates on properties of hot rolled steel in combination with different heating and rolling regimes.

Yet another object of the present invention is to develop a system for simulation of cooling which would be adapted to have laminar cooling from top and spray cooling from bottom surfaces of the rolled steel.

Yet another object of the present invention is to develop a system for simulation of cooling which would be adapted to study the effect of heat transfer additives on cooling rate of steel.

A further object of the present invention is to develop a system for simulation of cooling which would be adapted to study the effect of differential cooling rate on shape or internal stress of the rolled steel.

A still further object of the present invention is to develop a system for simulation of accelerated cooling which would be adapted to conserve water during the course of cooling.

SUMMARY OF THE INVENTION:

According to basic aspect of the present invention there is provided an ROT mounted accelerated cooling system designed for achieving a cooling rate of 5-30o C/s during cooling of steel sheet/plate of 2-20 mm thickness x 50-300 mm width rolled at a temperature of 900-1000o C.

According to another aspect the system includes water pumping unit for storage and supply of water for cooling purpose; top cooling unit connected to the water pumping unit through overhead water tank for extraction of heat from the hot steel using laminar flow of water; bottom cooling unit connected to the said water pumping unit through pressure relief valve for extraction of heat from the hot steel using spray jet of water; control unit for regulating flow rate of water in top and bottom cooling units; and arrangement for recirculation of cooling water for its conservation during study.

According to another aspect the said water pumping unit comprises of a concrete sump (having capacity of 3-4 m3) for storage of water; a centrifugal pump motor set for drawing requisite amount of water (typically 150-200 m3/hr at max 20 m head) from the sump and feeding to top and bottom cooling units; a strainer and simplex filter set for filtration of water to 100-200 micron level.

According to a further aspect the cooling system further comprises a top cooling unit includes an overhead steel water tank of 2-3 m3 capacity and placed at a height such that the water head is suitable for achieving laminar flow when discharged through a series of (typically 10 number) headers fitted with gooseneck delivery tubes (around 20 number per header). The maximum cooling area is typically of 2 m length and 300 mm width suiting to ROT design.

According to yet another aspect the cooling system, further comprises of a bottom cooling unit which includes series of (typically 10 number) spray header fitted with nozzles and placed between rollers of the ROT such that the spray jet of water impinges on bottom surface of the rolled steel at pressure (typically 0.5-1.5 bar) controlled through a pressure relief valve fitted in the delivery pipeline of the water pump. Similar to the top unit the maximum cooling area is typically of 2 m length and 300 mm width as per available space in the ROT.

According to a further aspect the cooling system also includes control unit consisting of manual valve with each header for their individual selection and motorized valve for each top and bottom cooling unit for proportionate opening or closing of the flow line in order to change the flow rate as per requirement. It also includes a pressure control loop with bypass and overflow lines. Operation of the motorized valves is from control desk of the mill. It also has provision of linking the operation from programmable logic controller (PLC) based control system of the mill. Provisions are also made for measurement of water pressure and steel temperature before and after the cooling with scope of recording the data in the mill’s data logging system.

According to another aspect the cooling system is provided with a water collecting tray at the bottom of the cooling headers so that used water is taken into the water sump for its recirculation in the system and thus conserving it during the study.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES:

Figure 1 shows photograph of the laboratory rolling mill in hot mode where the present invention has been carried out.

Figure 2 shows photograph of run-out-table (ROT) of the above laboratory mill before mounting the invented cooling system over it.

Figure 3 shows a schematic layout of the experimental cooling system along with the laboratory rolling mill and it’s ROT.

Figure 4 shows photograph of water pumping unit in accordance with the present invention.

Figure 5 shows photograph of overhead tanks connected with water pumping unit in accordance with the present invention.

Figure 6 shows photograph of top cooling header and associated items in accordance with the present invention.

Figure 7 shows schematic drawing of top cooling header in accordance with the present invention.

Figure 8 shows schematic drawing of bottom cooling header in accordance with the present invention.

Figure 9 shows photograph of bottom cooling header placed in the ROT in accordance with the present invention.

Figure 10 shows photograph of complete cooling system installed in the laboratory mill in accordance with the present invention.

Figure 11 shows photograph of laminar flow of water during measurement of flow rate from one of headers in accordance with the present invention.

DETAILED DESCRIPTION

Thus the present invention discloses a system for simulation of accelerated cooling of hot rolling mills. The present system basically includes means for storing and pumping cooling water at pre-set flow rate and pressure through series of top and bottom cooling headers placed respectively over and under the ROT of the laboratory mill to cool the hot rolled steel at desired cooling rate through laminar flow of water from top and spray jet of water from bottom of the steel plate/sheet. The system of the present invention is particularly designed to provide variable cooling rate suiting to requirements of different steels for achieving desired mechanical and metallurgical properties.

Reference is first invited from the accompanying Figure 1, which illustrates a preferred embodiment of the laboratory rolling mill in hot rolling mode. The mill includes a reheating furnace for heating and soaking of steel at set temperature and a reversing mill stand for deforming the steel in few numbers of passes at the desired temperature, strain, and strain rate with the help of drive system, screw-down system, PLC based control, monitoring and data acquisition system, etc. The mill is also equipped with proper instrumentation system for measurement of roll force, speed, torque, temperature, etc. and a control desk with touch type human-machine-interface (HMI). Further salient details of this laboratory mill in hot mode are given in Table 1.

Table 1

Max. reheating temperature 1300 oC
Mill configuration 2-Hi, 400 mm wide, reversing
Work roll diameter 350 mm
Max speed 30 m/min
Max load 150 ton
Max torque 5.2 t-m
Max size of input feedstock 100 mm thick x 300 mm wide
Min finished thickness 2 mm
Steel to be rolled All grades
Other feature It can also be used for cold rolling in 4-Hi mode

This conventional laboratory rolling mill has roller tables on both sides of the mill stand for transfer of material. The table on entry side of the mill between reheating furnace and mill stand is used for transporting hot feedstock from reheating furnace to the mill stand for rolling. The table on exit side of the mill is called run-out-table (ROT) and is used for transporting and cooling of the rolled material as shown in Figure 2. Since this ROT was not provided with any forced cooling system, the rolled steel was cooled in air only at very low cooling rate. Sometimes when high cooling rate was required, the rolled steels were quenched in water in a separate tank. There was no provision on this ROT to have varying cooling rates in between two extreme cooling rates achievable in air and quench medium (water), which was very much required to achieve desired properties of steel hot rolled at different heating and rolling regimes.

It presented an opportunity to develop an experimental accelerated cooling system and to mount on the ROT to achieve the varying cooling rates in a wide range typically from
5o C/s to 30o C/s. A preferred embodiment of the present cooling system is illustrated in

the accompanying Figure 3. As shown in this figure, the present cooling system basically comprises of cooling water pumping unit, top laminar cooling unit, bottom spray jet cooling unit and arrangements for control of water flow and pressure along with water recirculation circuit.

In a preferred embodiment of the present invention, as shown in Figure 3 and Figure 4, the water pumping system includes a concrete sump of 3-4 m3 capacity for storage of water, a centrifugal water pump with suitable water distribution network for supplying water at a specified flow rate and pressure (typically of 150-200 m3/hr at max. 20 m head) to top and bottom cooling units. The water pumping unit preferably includes a strainer and filter set to control the cleanliness level of supply water to 100-200 micron level. After the filter, total flow from the pump is equally divided into two parts, one each for cooling of rolled steel from top and bottom sides.

Reference is now invited to Figure 5, which shows that the top cooling unit includes an overhead steel tank which has a capacity of 2-3 m3 and is fitted with level indicator, water overflow line and drain line. This tank is connected to the water pumping unit for constant supply of water to a series of (typically 10 number) top cooling headers as

shown in Figure 6. The tank is placed at a height such that constant water head suitable for achieving laminar flow of water from the headers is maintained. In a preferred embodiment of the present system, all the top headers are fitted with gooseneck delivery tubes (around 20 numbers per header), as shown in Figure 7, to aid in achieving laminar flow of water. The maximum cooling area covered by the laminar flow of water is typically of 2000 mm long and 300 mm wide as per available space in the ROT.

As described earlier in Figures 3 and 4, the water for bottom cooling of rolled steel is drawn from the same water pumping unit and it is distributed to a series of (typically 10
No.) spray headers. Each header is fitted with 7-8 nozzles as shown in Figure 8. These headers are placed between rollers of the ROT, as shown in Figure 9, such that the spray jet of water impinges on bottom surface of the rolled steel at a set pressure (typically 0.5-1.5 bar), which is controlled through a pressure relief valve fitted in the delivery pipeline of the water pump. Similar to the top unit, the maximum area covered by the spray of water is typically of 2000 mm long and 300 mm wide as per available space in the ROT.

In a preferred embodiment of the present invention, suitable features have been added in the cooling system for control of flow parameters of the cooling water. With this system the cooling rate can be varied either through varying flow rate of water or through selection of particular sequence of cooling headers at top and bottom. Accordingly, one motorised valve has been provided in each cooling unit to vary flow rate of water from 0-100%. Similarly manual valve has been provided in each header to select or deselect any particular header. As described earlier, the laminar flow of water is achieved for top cooling unit through control of pressure using fixed water head in the overhead tank. Spray pressure for bottom cooling unit is controlled through pressure reducing valve placed after the pump. It also includes a pressure control loop with bypass and overflow lines. Operation of the motorized valves is from control desk of the mill. It also has provision of linking the operation from PLC based control system of the mill. Provisions are also made for measurement of water pressure and steel temperature before and after the cooling with scope of recording the data in the mill’s data logging system.

TESTING:

As shown in Figure 10, the innovated cooling system was installed and commissioned at the laboratory mill for carrying out experiments with different heating, rolling and cooling rates of steel. It also shows that a water collecting tray was provided at the bottom of the cooling headers so that used water was taken into the water sump for its recirculation in the system. It helps in conserving water during different studies.

In order to test functioning of the system, initially cold trials were carried out to evaluate flow rate of water from each header, as shown typically for one header in Figure 11. The measured value of flow rate matched with the designed flow rate. Working of complete system was checked by running it offline (without hot rolling) to ascertain effectiveness of flow control, pressure control, operational ease, etc. and it was found satisfactory.

Finally hot rolling of steel was carried out at different temperatures and in different sizes and the rolled materials were cooled using the innovated experimental cooling system. It validated that the cooling rate of steel was in the range from 5o C/s to 30o C/s as specified during the design stage.

Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.