Abstract:
The hot strip mill for rolling slabs of a minimum thickness on the order of 7.75 inches into strip on the order of 1000 PIW comprises a plurality of mill stands TM1 through TMx, each of the stands spaced from an adjacent stand by a distance less than the length of the strip between the stands so as to roll in tandem at a constant mass flow. The method of rolling includes reducing slabs into the strip thickness through continuous passes on the TM1 through TMx mill stands while maintaining a constant mass flow on each stand and a minimum temperature differential from head to tail. The method includes selecting the correct slab thickness to achieve the desired productivity and temperature differential.

Description:
FIELD OF THE INVENTION 
     My invention relates to hot strip mills and, more particularly, to continuous hot strip mills for reducing slabs to strip thicknesses, the slabs being of such size as to provide coils on the order of 500 to 1000 PIW and greater 
     DESCRIPTION OF THE PRIOR ART 
     Conventional hot strip mills have consisted of a roughing train and a finishing train separated by a holding table to accommodate the transfer bar out of the roughing train and direct that transfer bar into the finishing train at the desired suck-in speed. It has been recognized that the transfer bar loses heat through radiation on the holding table and its heat loss increases as the thickness of the transfer bar decreases. It is also known that there is a temperature differential from front to tail of the product being rolled which temperature differential can affect metallurgical properties of the product and loading requirements of the mill stands. While the slab may be uniformly heated in a reheat furnace, this temperature differential exists because there is a time lapse between when the front end of the slab first enters the hot strip mill and when the tail end of the slab enters the mill. 
     A number of solutions have been employed to minimize heat loss through radiation and decrease this front-to-tail temperature differential. For example, coil boxes have been provided to hold the transfer bar in coil form prior to introduction to the finishing train. Tunnel furnaces have also been employed over the holding table so that the transfer bar is maintained at the appropriate temperature. Another attempt to solve this problem has been through the utilization of an intermediate mill having coiling furnaces on either side of the reversing mill. While all of these solutions have been successful in varying degrees, there still remains a need for a mill which can handle slabs of such size as to provide the greater PIW coils required in today&#39;s market without excessive auxiliary equipment yet still maintain acceptable temperature differentials so as to provide uniform metallurgical properties and not unduly load the individual mill stands. 
     Previous attempts to provide a true continuous hot strip mill with all stands arranged in tandem for straight-through rolling have been unsuccessful. It is thought that such attempts did not work for there was no recognition of the radiation losses for the slab thicknesses employed. These early attempts involved utilizing slabs on the order of two inches thick and rolling them through a series of stands in a way that is comparable to passing a transfer bar through a finishing mill today. In addition, it has been believed that it is necessary to maximize rolling speeds in the roughing mill and then hold the slab prior to entering the finishing train at an appropriate suck-in speed for continuous finishing on the tandem finishing stands. 
     SUMMARY OF THE INVENTION 
     My invention completely eliminates the transfer bar as it is presently known and further eliminates the holding table as it is presently known. Further, my invention greatly reduces the temperature differences between the front and tail of the slab and resultant strip product by continually reducing the slab at a constant mass flow for each mill stand. Further, my invention avoids excessive temperature loss through radiation by eliminating the discontinuity in processing resulting from the existing holding table. 
     All of this is accomplished while greatly reducing the length of the mill and minimizing the auxiliary equipment utilized heretofore. Finally my invention permits slabs to enter the continuous hot strip mill at temperatures as much as 400° F. less than the temperatures presently employed in existing mills. This translates into a tremendous energy savings and costs associated therewith. 
     My invention is a continuous tandem hot strip mill for rolling slabs of a minimum thickness on the order of 7.75 inches into strip thicknesses, the coils of which are on the order of 500 to 1000 PIW and greater which comprises a plurality of mill stands TM1 to TMx with each of the stands being spaced from an adjacent stand by a distance less than the length of the strip between the stands so as to roll in tandem therewith at a constant mass flow. 
     I have found that for a desired temperature front-to-tail differential and a given set of production requirements, i.e., cycle time, it is possible to determine a minimum critical material thickness (h) for entering TM1. The thickness is obtainable from the relationship α T  =f(h,T) and preferably from the empirical relationship ##EQU1## where αT is the temperature loss rate at the temperature T, ΔT represents the acceptable front to tail strip temperature differential; T F  is the front end temperature of the slab entering TM1; α=2.9/h 105  is the temperature loss rate at 1800° F. in °F./sec.; n=0.0025/(1+0.1h) is a parameter defining the variation of α with temperature in °F. -1  ; and t is the time interval between the moment when the slab front end enters TM1 and the moment when the slab tail end enters TM1. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic showing the general arrangement of a conventional continuous hot strip mill; 
     FIG. 2 is a schematic showing the general arrangement of an existing modernized hot strip mill employing a tunnel furnace; 
     FIG. 3 is a schematic showing the general arrangement of my invention; 
     FIG. 4 is a graph showing temperature loss rate due to radiation as a function of material thickness and temperature; and 
     FIG. 5 is a graph showing the effect of material thickness entering the tandem mill in relation to the difference in temperature between front and tail ends of the slab. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The hot strip mill of FIG. 1 is an existing conventional hot strip mill comprised of a roughing train comprised of mill stands R1-R5 with appropriate vertical edgers and scalebreakers and a finishing train comprised of tandem mill stands F1-F6 with appropriate crop shear and scalebreaker. The hot strip mill receives slabs which have been reheated in one of the four furnaces provided. The roughing train is separated from the finishing train by a holding table in excess of 200 feet. A slab is reduced to a transfer bar in the roughing train and then retained on the holding table prior to being fed into the finishing train defined by the mill stands F1-F6. The transfer bar is rolled continuously and in tandem to strip thicknesses on the finishing train. At the exit end of the last finishing stand F6 there is a long runout table which employs cooling water sprays to cool the strip down from the finishing temperature to the desired temperature prior to being coiled on one of three downcoilers. It can be seen that the total length of the hot strip mill from the first roughing stand R1 to the last finishing stand F6 is in excess of 600 feet. 
     One solution to reducing the length of the mill while providing the necessary temperature differential from front to tail of the coil has been through the utilization of a tunnel furnace on the holding table, FIG. 2. This modernized hot strip mill includes three reheat furnaces and two roughing mill stands R1 and R2 which comprise the roughing train. The holding table is on the order of 190 feet and is covered by an appropriate tunnel furnace. The tunnel furnace purportedly equalizes temperature and reduces front-to-tail transfer bar temperature differential. The finishing train preceded by an appropriate crop shear and scalebreaker includes six mill stands F1 through F6 where the strip is rolled continuously and in tandem. A runout table and downcoiler similar to that illustrated in the embodiment of FIG. 1 follows the last finishing stand F6. The length of the hot strip mill of FIG. 2 is less than that of FIG. 1 and is on the order of 490 feet. 
     My hot strip mill is illustrated in FIG. 3. Three furnaces are illustrated for reheating the slabs to the appropriate temperature. As will be seen hereinafter, the temperature of the slab entering my hot strip mill is on the order of 1800° to 1850° F. which is 400° to 500° F. less than in existing mills. Such a reduced initial temperature makes my hot strip mill adaptable for receiving slabs from a continuous slab caster as well as from reheat furnaces. The mill itself is comprised of nine stands identified as TM1 through TM9. Appropriate vertical edgers are provided before the initial stands TM1 through TM4 and a crop shear is provided between TM4 and TM5. The length of the mill from the first vertical edger through the last stand TM9, is only on the order of 200 feet which is severalfold less than for existing mills as well as modernized mills. 
     The key to my mill is that the mill stands TM1-TM9 are spaced so that the entire rolling is continuous and in tandem while a constant mass flow is maintained through each rolling mill stand. This constant mass flow is expressed as h i  ×V i  =constant, where h i  is the exact thickness out of the stand and V i  is the actual mill stand speed. 
     Because the front end and the tail end of the slab enter the tandem mill stands at different moments of time, there is an initial temperature differential between the two ends even though the slab is evenly heated. Tjis temperature differential is due to the different time during which the front and tail ends are subjected to heat radiation and convection. 
     This temperature loss rate (α T ) is basically a function of the material thickness (h) and temperature (T), i.e. 
     
         α.sub.T =f(h,T)                                      (1) 
    
     A typical plot of the Equation (1) is shown in FIG. 4. Therefore the temperature differential between the front and tail ends (ΔT) may be calculated as follows 
     
         ΔT=α.sub.T ·t                         (2) 
    
     where t is the cycle time, or the time interval between the moment when the front end enters the tandem mill and the moment when the tail end enters the tandem mill. 
     The cycle time is equal to ##EQU2## where PIW=the rolling material weight per inch of width (lb./in.), 
     TPH=the mill production, short tons/hr. 
     W=the rolling material width, in. 
     The rolling characteristics of the material and also its metallurgical properties will be uniform when ΔT is minimum. Practices from the best operated hot strip mills show that ΔT is satisfactory when: 
     
         ΔT≦30° F.                              (4) 
    
     Now knowing the cycle time (t) and the material temperature (T F ) when entering the tandem mill, the critical material thickness h CR  to satisfy the Equation (4) can be defined. 
     For 1000 PIW and W=40 in. and 800 TPH, I determine from Equation (3) ##EQU3## Then from Equation (2) and Equation (4) I determine ##EQU4## Referring to FIG. 4, I determine that 
     
         h.sub.CR =7.86 in. 
    
     It should be noted that Equations (1) and (2) are valid when the material temperature is constant. 
     In fact, the temperature is decreasing with time. This temperature decay is taken into account in the following equation. ##EQU5## where T F  =front end temperature when entering the mill, °F.; e is the logarithmic base; α=temperature loss rate at 1800° F., °F./sec.; and n=parameter defining the variation of α with temperature, °F. -1 . α in turn is 
     
         α=2.9/.sub.h 105                                     (6) 
    
     and ##EQU6## 
     The Equations (5) through (7) are plotted in FIG. 5 for the cycle time of the earlier example. 
     From FIG. 5 we can compare performance characteristics of the conventional HSM, the existing modernized HSM and my invention. 
     The material thickness h entering the tandem finishing train in the conventional hot strip mill (FIG. 1) is within the following range: 
     
         0.75≦h≦1.5 in.                               (8) 
    
     For some hot strip mills (FIG. 2) built or modernized in the late 70&#39;s, the range was shifted to: 
     
         1.8≦h≦3.15 in.                               (9) 
    
     Finally, the material temperature when entering the tandem finishing train for existing mills is normally above 1800° F. with the slabs exiting the furnace for introduction into the roughing mill at 2250° F. 
     As it follows from FIG. 5, the condition (5) is not satisfied for the range (8) or for the range (9). To compensate for an excessive temperature drop, a number of different solutions have been suggested including the coil box, an additional stand preceding the tandem mill and the tunnel furnace installed between roughing and finishing trains, also acceleration of the mill, etc. This results in further complication of the installation, operation and maintenance of the hot strip mill. 
     However, it can be seen from FIG. 5 that the material thickness h must exceed a certain critical value h CR  as expressed below. 
     
         h&gt;h.sub.CR                                                 (10) 
    
     In other words, when h&gt;h CR , the condition (4) will be satisfied without any additional measures mentioned above. The magnitude of h CR  depends on the slab length (or the slab weight per inch of width), the slab temperature and the rolling cycle time. For a slab with 1000 PIW and cycle time equal to 90 seconds we obtain h CR  =7.75 in. 
     Thus, if a 7.75 inch thick slab at 1800° F. is entered into my tandem mill, the front-to-tail temperature differential of the finished product will be no more than 30° F. In reality, the higher temperature dissipates faster than the lower temperature and, therefore, the temperature differential continues to diminish as the strip travels through my mill. 
     From the relationship between the transfer bar thickness and front and tail end temperature differential illustrated in FIG. 5, it can be seen that for the conventional hot strip mill of FIG. 1 and for the existing modernized hot strip mill of FIG. 2, the transfer bar thicknesses entering the finishing train are located at the end of the curves which result in high front-to-tail temperature differentials and which thus require higher initial slab temperatures as well as auxiliary equipment such as zooming, tunnel furnaces and the like. On the other hand, it can be seen that the Tippins constant mass flow hot strip mill will provide a front-to-tail temperature differential on the order of 30° F. for slabs entering the mill at 1800° F. at a thickness of 7.75 inches and greater without the need for any such auxiliary equipment. 
     Therefore, as long as one knows the requirements for PIW, ΔT and the width of the product which is normally based on a weighted average of the product mix and the TPH production requirements, the given minimum critical slab thickness can be readily determined from the Equations (5) through (7), or the respective curves such as FIG. 5. 
     The following Table 1 is a rolling schedule and temperature profile for the rolling of a slab into strip thicknesses on my continuous tandem hot strip mill. The slab of low carbon steel has a thickness of nine inches, a width of 39.5 inches and a length of 32.72 feet. The temperature out of the furnace is 1850° F. and the final strip thickness is 0.111 inch. 
     
                                           TABLE 1__________________________________________________________________________Rolling Schedule and Temperatures          Mass    Mill  Flow  TemperatureGauge    Speed (h.sub.i V.sub.i)                Entry  Exit   Rated                                  ReductionMill (h.sub.i) in.    (V.sub.i) FPM          in. × FPM                Front                    Tail                       Front                           Tail                              H.P.                                  %__________________________________________________________________________Furnace9.000    --    --    1850                    1850                       1850                           1850                              --  --VE   9.000     21.6 194.3 1844                    1817                       1810                           1782                              1500                                  --TM1  7.000     27.8 194.3 1798                    1771                       1794                           1768                              1500                                  22.2TM2  5.000     38.8 194.3 1770                    1744                       1734                           1709                              2500                                  28.6TM3  3.000     64.8 194.3 1711                    1687                       1715                           1691                              5000                                  40.0TM4  1.250    155.4 194.3 1692                    1669                       1705                           1683                              10000                                  58.3TM5  0.600    323.8 194.3 1682                    1660                       1661                           1640                              6000                                  52.0TM6  0.3300    588.6 194.3 1648                    1627                       1659                           1639                              6000                                  45.0TM7  0.205    946.6 194.3 1645                    1626                       1654                           1636                              6000                                  37.9TM8  0.138    1407.6          194.3 1640                    1623                       1647                           1630                              6000                                  32.7TM9  0.111    1750.0          194.3 1634                    1617                       1634                           1619                              4000                                  19.6__________________________________________________________________________ 
    
     It can be seen that providing constant mass flow and exiting TM9 at temperatures on the order of 1617°-1634° F. requires an entrance speed into the initial stand TM1 of only 27.8 ft./min. and subsequent speeds through TM3 of only 64.8 FPM. Heretofore it has been the practice to enter the roughing train at much higher speeds. Yet the subject mill has a peak productivity of 781.7 TPH or 4 million tons per year which compares favorably with existing mills. 
     The temperature differential of the final product out of TM9 is on the order of 17° F. and the initial slab temperature was only 1850° F. This has been achieved without the benefit of any zoom or auxiliary equipment or supplemental heating. 
     It can, therefore, be seen that I have a provided a mill where there is no discontinuity in process resulting in additional temperature loss. In addition, the entire mill is operating at a constant mass flow and an optimum speed for a given slab thickness. Therefore, the operation is simplified and because of the tremendous decrease in slab temperature out of the furnace, tremendous conservation of energy has also been achieved. I have found for every cycle time there is a critical material thickness entering the continuous tandem mill which provides the acceptable temperature differential from front to tail to achieve uniform metallurgical properties and acceptable rolling conditions.