THE OPERATION OF A MULTI-STAND HOT ROLLING MILL

Abstract: A multi-stand hot rolling mill has provision for interstand cooling and, in order to obtain rolled strip with the required profile, the amount of interstand cooling is changed during rolling to bring about a change in rolling load at one or more of the stands to thereby influence the strip profile from said one or more of the stands.

Description: THE OPERATION OF A MULTI-STAND HOT ROLLING MILL This invention relates to a method of operating a multi-stand rolling mill for rolling hot metal strip. In the manufacture of metal strip, it is usual to reduce a hot metal orkpiece down to strip by passing it, while hot, through the stands of a finishing mill. There are usually at least six stands arranged in tandem and the hot workpiece is progressively reduced in thickness as it is passed through the stands. To ensure that the strip, assuming it to be of steel, has the required metallurgical properties, it is necessary for the target finishing temperature of the strip as it leaves the last stand to be typically 870°C. In a conventional hot strip mill with a delay table between the roughing and finishing mills, there is a considerable temperature differential between the head and tail ends of the workpiece as each enters the first stand of the finishing mill and care has to be taken to ensure that the temperature of the strip along its length is at, or close to, the target finishing temperature on exit from the last stand. Furthermore, the consequence of this falling entry temperature is that the rolling loads at the mill stands increase as the workpiece is rolled, even when the mill is accelerated to keep the finishing temperature constant. As a result, the rolls of the stands tend to deflect progressively as the workpiece passes through the mill and this leads to a change in strip profile. By the expression "strip profile" is meant the variation in thickness of the strip between its centre and positions close to its lateral edges. The mill has to be operated at a relatively slow speed, known as the threading speed, as the head of the workpiece passes through the stand and is attached to a coiler located downstream of the last stand and, thereafter, the speed of the mill is increased to its rolling speed. It is disclosed in the specification of European Patent No. 0024849 to operate a multi-stand hot rolling mill where, during threading, the rolling load at each stand is set to ensure that the required output profile at the last stand is obtained and the rolling temperature at the last stand is at, or close to, a predetermined level. The speed of rolling is increased and cooling liquid is applied to the workpiece at one or more interstand locations to cool it, such that the rolling temperature at the last stand remains at, or close to, the predetermined level. At each interstand location where the coolant is applied, the quantity of coolant is such that the rolling load of the next stand downstream of the location remains substantially equal to that set up during threading. However, in certain circumstances, it may not be desirable to keep the rolling load of each stand substantially equal to that set up during threading. If the profile of the strip is detected to have changed from what is desired, it is necessary to be able to quickly change the rolling load on one or more of the stands in order to influence the profile of the strip leaving that stand. An object of the present invention is to provide a means of controlling finishing temperature along the length of the workpiece and, at the same time, to provide a means for controlling strip profile along the length of the workpiece. According to the present invention, in a method of operating a multi-stand hot rolling mill, the head end of a hot metal workpiece is threaded at a relatively slow speed through the stands of the rolling mill and the rolling load at each stand is set to ensure that the required profile of the strip at the last stand is obtained and the rolling temperature at the last stand is at or close to a predetermined level; the speed of rolling is then increased and cooling liquid is applied to the workpiece at interstand locations to cool it, such that the rolling temperature at the last stand remains at or close to said predetermined level; and, by changing the amount of interstand cooling during rolling of the workpiece, a change in rolling load at one or more of the stands is brought about thereby influencing the strip profile from said one or more of the stands. The interstand cooling may be changed between forward and reverse modes during rolling of the workpiece. The amount of liquid coolant applied at the or each interstand location may be progressively increased to a maximum prior to liquid being applied at the adjacent interstand location. Interstand cooling in the forward mode comprises applying liquid coolant between a pair of the early stands of the mill and then, when a predetermined flow is achieved, applying coolant at the next interstand location in the downstream direction and so on. Interstand cooling in the reverse mode comprises applying liquid coolant between a pair of the later stands of the mill and then, when a predetermined flow is achieved, applying coolant at the next interstand location in the upstream direction and so on. The liquid coolant may be applied to only a part of the width of the strip at the or each location. The invention can also be used to control strip profile in conjunction with work roll bending devices and/or axially movable work and intermediate rolls. The latter are essentially a mill set-up function, whereas work roll bending can be used dynamically down the workpiece-length to counteract the influence of variations in rolling loads on strip profile. Where roll bending is employed, it is a purpose of this invention to give the roll bending a greater range control in initially setting up the head end bending loads by relieving the work roll bending system entirely, or partially, of the need to respond to variation in rolling loads down the length of the workpiece. This is brought about by modifying the normal amount of change in rolling load by varying the manner in which the interstand cooling is applied progressively down the workpiece. The interstand cooling may be used in conjunction with roll coolant sprays where the water flow remains constant across the width of the workpiece and is not varied down the length of the workpiece, or it may be used in conjunction with roll cooling sprays which can have their flow rate varied across the width of the workpiece in order to influence work roll thermal camber and also be varied down the length of the workpiece to influence thermal camber growth. The invention can apply equally to mills employing at the ingoing side (1) an open delay table; (2) to mills using a heat shield such as ENCOPANELS surrounding the workpiece on the delay table; (3) to mills employing a STELCO coilbox. It will be appreciated that, with mills employing either ENCOPANELS or a STELCO coilbox, the temperature differential between the head and tail ends of the workpiece as it enters the first stand Fl is less than on mills where the delay table is unshielded. As a result, such mills must use more interstand cooling to achieve a given rate of acceleration, but these mills also tend to achieve more constant temperature and rolling loads down the workpiece length. For the purpose of describing the invention, reference will be made to Figure 1 of the accompanying drawings which shows a hot strip mill employing an ENCOPANEL system 1 on its delay table 3 and seven finishing stands Fl to F7. Means for supplying variable flow interstand curtains 5 of liquid coolant are located downstream of stands Fl to F6, inclusive. Each interstand curtain 5 is in two parts engaging respective opposite faces of the workpiece. Those downstream of Fl and F2 can be used, if desired, to suppress the formation of secondary scale by applying of the order of 20% of their maximum flow rate to chill the strip surface enough to slow down the rate of oxidation. The remaining curtains can be used to control finishing temperature by variation of the flow rate to compensate for the effect of variations in incoming workpiece temperatures and variations in rolling speed. A profile detector 6 is located downstream of the last stand. At the head end of the workpiece, it may be decided to apply the initially required amount of cooling water by first bringing in either the pair of curtains after stand F5 or the curtains after F3. In either case, more curtains are brought in as extra water is required during mill acceleration. Where very high rates of acceleration are required, it may be necessary, to achieve correct finishing temperatures, to increase the flow rate of the curtains downstream of Fl and F2 to amounts greater than those needed simply for scale suppression. In order to describe the invention, it will be assumed that all six pairs of curtains are used for temperature and load control. The amount of water needed downstream of Fl and F2 for scale suppression will be ignored for the purpose of example. It will be readily understood that the method of application of water could be either "forward mode" in which water is first applied downstream of Fl cascading towards F6 or "reverse mode" in which water is first applied to one of the downstream stands, such as F5, and then cascading towards Fl. The choice of method, whilst giving correct finishing temperature in both instances, can influence rolling load and roll bite temperature considerably, as shown by the results set out in the Table of the following Example.- EXAMPLE Consider a low carbon steel workpiece, 261' long, 58" wide and 1.277" thick, passing through the ENCOPANEL system and reaching the finishing mill with the front end at 1045°C and the tail end at 1029°C. This is typical of what can be achieved by a 288' long ENCOPANEL system with all the panels in use. The workpiece is progressively reduced in the seven finishing stands to 0.100" to give a temperature at the finishing pyrometer of 870°C. In order to achieve this, the front end is threaded at 2249 fpm and, after some 21' of the workpiece have been rolled, the mill is accelerated to reach a tail end speed of 2908 fpm. Without interstand cooling, such a schedule would give a front end finishing temperature of 883°C and a tail end temperature of 907°C but, by applying approximately 5,800 litres of cooling water per minute after Fl at the head end and a total of 21,000 1pm after stands Fl, F2 and F3 at the tail end, the correct 870°C finishing temperature is achieved. However, the strip profile may not be satisfactory. The rolling loads, strip temperatures and required water quantities at each stand for the head and tail ends for cooling in both forward and reverse modes are set out in the following Table :- TABLE Reverse Mode Entry lpm Temp. Load lpm tons 5800 Fl 1004 2298 9000 F2 997 2078 9000 F3 946 1967 3000 F4 935 1668 F5 923 1396 F6 907 1098 F7 888 868 Pyro 870 It can be seen that reverse mode cooling, where coolant is first applied after stand F5 and when maximum flow is reached additional cooling is applied after F4, produces the same outgoing temperature as forward mode cooling but the loads on some of the stands are reduced. The range of rolling load control at each stand, depending on the mode of water application, is, therefore, the number of tons shown below:- Fl F2 F3 F4 F5 F6 F7 Head 0 77 68 60 49 0 0 Tail 0 101 189 188 90 0 0 It will be observed that the loads in F6 and F7 are not changed by varying the mode of water application and, as a result, the strip profile will not be influenced at these two stands. However, the range of control at F2 to F5 is large enough as a proportion of the mill load to influence strip profile leaving F5 to a major degree. Changing from reverse mode to forward mode progressively would increase rolling loads down the workpiece by the percentages as set out belo :- Fl F2 F3 F4 F5 F6 F7 4.5 8.2 13.4 14.6 13.9 8.3 7.7 Changing from forward mode to reverse mode would change rolling loads by the percentages shown below:- Fl F2 F3 F4 F5 F6 F7 +4.5 -1.1 -1.4 -2.4 +2.4 +8.3 +7.7 It is , therefore, possible to choose the amount of rolling load increase to lie anywhere between these two limits in order to control strip profile. In a stand, a change of 100 tons in the rolling load will typically alter the strip profile by 10 um. Thus the range of change of profile in μm out of each stand that can be obtained in this example by switching modes of coolant is shown below:- Fl F2 F3 F4 F5 F6 F7 Head 0 7.7 6.8 6.0 4.9 0 0 Tail 0 10.1 18.9 18.8 9.0 0 0 In addition, where roll coolant flow is controllable, it is possible to select a rate of change of roll coolant down the workpiece so as to achieve a change of work roll thermal profile such that the effect of the interstand cooling application strategy on strip profile is within the desired limits. Depending upon the particular rolling schedule and the particular amount of acceleration required, it will be possible to control strip profile either just by changing the mode of application of interstand cooling or by doing this whilst, at the same time, changing the amount of roll coolant application. The use of the two methods in conjunction could allow a higher rate of acceleration to be employed without running out of strip profile control capability. It is possible to provide cooling immediately upstream of the first stand and immediately downstream of the last stand. Likewise, it would be possible to vary the effective width of the curtains to spray either a central portion or the whole width of the strip.

Claims: Claims : 1. A method of operating a multi-stand hot rolling mill in which the head end of a hot metal workpiece is threaded at a relatively slow speed through the stands of the rolling mill and the rolling load at each stand is set to ensure that the required profile of the strip at the last stand is obtained and the rolling temperature at the last stand is at or close to a predetermined level; the speed of rolling is then increased and cooling liquid is applied to the workpiece at interstand locations to cool it, such that the rolling temperature at the last stand remains at or close to said predetermined level; and, by changing the amount of interstand cooling during rolling of the workpiece, a change in rolling load at one or more of the stands is brought about thereby influencing the strip profile from said one or more of the stands. 2. A method as claimed in claim 1, characterised in that the interstand cooling is changed between forward and reverse modes during rolling of the workpiece. 14 3. A method as claimed in claim 2, characterised, in that the amount of liquid applied at the or each interstand location is progressively increased to a maximum prior to the liquid being applied at an adjacent interstand location. 4. A method as claimed in any preceding claim, characterised in that the liquid is applied in the form of a curtain extending over the width of the strip. 5. A method as claimed in any preceding claim, characterised in that the liquid coolant is applied to only part of the width of the strip at the or each location. 6. A method as claimed in any preceding claim, characterised in that the profile of the strip leaving the last stand is detected and the amount of interstand cooling is controlled as a result of said detected profile in the sense to change the profile of the strip leaving the last stand to a required form. 7. A method as claimed in any preceding claim, characterised in that liquid cooling is applied to the mill rolls. 8. A method as claimed in claim 7, characterised in that the liquid is applied to only a part of the length of the rolls. 9. A method as claimed in any preceding claim, characterised in that work roll bending is applied to work rolls of one or more of the stands.

Inventors: HEWITT EWAN CHRISTIAN, HEWITT, EWAN, CHRISTIAN
Applicants: DAVY MCKEE SHEFFIELD, DAVY MCKEE (SHEFFIELD) LIMITED
Application year: 1989
Main classification: B21B37
IPC codes: B21B1, B21B37, B21B45