Method of manufacturing hot strip

A method and apparatus for manufacturing hot strip steel includes a continuous slab caster adapted to cast a strand of 4 inches or more in thickness. The strand is cut to slab length and heated in a roller hearth furnace or a walking beam furnace prior to entry into a multi-stand tandem reversing mill. The slab is rolled in a downstream direction through the mill in a first reducing pass. The mill is reversed and the elongated slab is rolled in an upstream direction through the mill in a second reducing pass. The intermediate bar exits the upstream end of the mill and is coiled in a coil box positioned intermediate the furnace and the reversing mill. The bar is then unwound from the coil box for rolling in the downstream direction through the mill in a third and finish pass. The finish gauge strip moves down a run out table downstream from the mill for coiling in a down coiler apparatus.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the manufacture of hot rolled 
steel strip from continuous slab casters and, more particularly, to a 
method and apparatus for increasing the productivity of the mill. Use of a 
continuous caster greatly reduces the size and expense of the plant due to 
the elimination of ingot casting, reheating and blooming and slabbing 
mills. 
It is known to continuously cast steel into thin slab, on the order of 2 to 
21/2 inches (.apprxeq.50-65 mm) thick, and to subsequently cut and process 
the lengths of cut material directly to strip on a hot finishing mill 
train. A buffer, in the form of a furnace, is usually provided between the 
casting station and the rolling mill to keep the cast material hot prior 
to rolling and to accommodate the difference in casting and rolling 
speeds. The cast stand is cut into a desired slab length downstream of the 
caster and the slab is either maintained in an elongated flat shape or it 
is coiled and held in the furnace buffer prior to finish rolling. U.S. 
Pat. Nos. 4,829,656 to Rohde and 4,698,897 to Frommann, et al. are 
exemplary of these above-mentioned known processing techniques for 
producing hot strip to finish gauge from a continuous caster. 
It has been common practice to roll the cast slabs from the buffer furnace 
into finish hot strip on a multi-stand continuous finishing mill line. 
Single stand reversing finishing mills with coil boxes on upstream and 
downstream sides of the mill have also been proposed as noted in the 
aforementioned U.S. Pat. No. 4,698,897 to Frommann, et al. and also in 
U.S. Pat. No. 5,150,597 to Sekiya, et al. to further process the material 
into finish strip gauge. In U.S. Pat. No. 4,998,338 to Seidel, et al., 
thin continuously cast slab, on the order of 2-21/2 inches thick 
(.apprxeq.50-65 mm), is finish rolled in a 3 or 4 stand reversible 
finishing mill train with a coil box located at the downstream end 
thereof. The mill is supplied with metal from one or two continuous 
casters. The cast strands are cut into slab length after solidification 
and then heated in a soaking furnace to a selected rolling temperature 
prior to rolling in the reversible finishing train. 
There has been a trend in recent years toward the installation of compact 
mills where a thin slab caster is directly connected to a continuous hot 
finishing mill. While these proposed mill configurations offer many 
advantages over conventional continuous hot strip mills, such as reduced 
space requirements, lower installation costs and lower man-hours per ton, 
there are, nevertheless, a number of significant disadvantages present in 
these compact mills. Among such disadvantages are low productivity (about 
800,000 tons/year); and unbalanced production between the continuous 
caster and the finishing mill train (800,000 tons/year per caster and 
3,000,000 tons/year for the finishing mill). In addition, the continuous 
finishing trains are expensive and require high installed horsepower to 
reach thin finished gauges. A common final strip thickness of 2 mm is 
oftentimes difficult to obtain in these mills. Finally, these prior mills 
are limited to a relatively narrow range of steel grades. 
The present invention overcomes the disadvantages and shortcomings of the 
prior art by providing a compact hot strip mill and continuous slab caster 
which significantly increases the annual production over conventional 
strip slab casters. 
The present invention further provides a less complex mold design in the 
continuous caster and increases the number of steel grades which may be 
cast. In addition, the invention permits the production of shorter length 
slabs compared to conventional strip cast slabs, thus, making it easier to 
shift slabs when operating a multi-strand caster along with the 
possibility of employing shorter length soaking/buffer furnaces. 
Still further, the present invention provides a finishing mill 
configuration having fewer mill stands than conventional continuous strip 
mills with lower installed motor horsepower. The finishing mill component 
of the invention is supplied with a greater ton/hour output of steel from 
the caster to provide a more balanced production and take advantage of the 
heretofore greater output capabilities of the finishing mill. As a result 
of the higher output of the caster, the invention provides reduced 
bar-to-bar waiting time which allows the mill to run at steady state 
conditions for greater periods of time than heretofore possible. 
SUMMARY OF THE INVENTION 
These, as well as other advantages and objectives are provided by the 
present invention which, briefly stated, comprises a one or two strand 
continuous slab caster, an in-line furnace, an intermediate coiler, a 
multi-stand, reversible finishing mill train and a finish strip coiler 
downstream of the finishing mill. The continuous caster produces a slab 
strand or strands of medium to moderately heavy thickness, for example 
from about 4 inches to about 6.5 inches (100-170 mm) in thickness. 
Preferably, the cast slab is about 4 inches in thickness. If cast slab has 
a thickness greater than 4 inches, it is preferable to use an 
intermediate, one-way roughing mill stand to reduce such heavier material 
to about 4 inches in thickness prior to introduction into the finishing 
mill train. 
In either event, the solidified cast strand(s) from the caster is cut to a 
desired slab length of, for example, about 85 feet (26 meters) downstream 
of the caster and prior to entry into an in-line roller hearth furnace. 
The cut slabs advance through the furnace, while being heated to a proper 
temperature in preparation for rolling. 
The finishing mill train preferably comprises four reversible mill stands. 
The last, or fourth mill stand at the downstream position, preferably 
remains idle in all passes except for the final, third finish pass, so as 
to improve strip surface quality and prolong the roll life of the work 
rolls of the last finish mill stand. In operation, a preheated slab enters 
the finishing mill train and is reduced in a first pass in the first three 
mill stands to a thickness of about 13/8-13/4 inches (35-40 mm) and 
concurrently elongated to a length of about 230 feet (70 meters). The 
rolls of the finishing mill are then rotated in a reverse direction to 
carry out a second pass in an upstream direction to reduce the material to 
a thickness of about 5/16-9/16 inches (8-15 mm). The greatly elongated 
material from the second rolling pass is taken up in an intermediate 
coiler, preferably an up coiler, located between the furnace and the 
finishing mill. The material is then withdrawn from the intermediate 
coiler for the third and last pass through all four of the stands of the 
mill traveling in the downstream direction. The rolls of the fourth mill 
stand engage the material to establish a finish gauge. The finished strip 
having a minimum thickness in the range of 0.060-0.080 inches (down to 1.5 
mm) moves along a run out table to a down coiler for coiling. Finished 
coils, on the order of about 30 tons each, are typically produced.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, FIGS. 1-4 schematically depict an apparatus 
and method according to the present invention. The hot strip mill 
according to the present invention generally designated 2 in the drawings 
is situated downstream from a continuous slab caster 4. The slab caster 4 
produces a continuous steel strand 5 having a thickness on the order of 
about 100 mm or 4 inches in thickness. A conventional cutting apparatus 
such as a torch cutter 8 cuts the cast strand 5 at predetermined 
intervals. The strand is cut into slabs 7 of a desired length determined 
by the required finish coil size. A typical length for a 4 inch thick slab 
may be about 85 feet (26 meters). 
A conventional roller support system 9 conveys the cut slabs 7 to an 
in-line, roller hearth furnace 6 for heating the slabs to a prescribed 
rolling temperature. The roller hearth furnace 6 contains sufficient 
accumulation space to permit from about 5 to 10 minutes of mill outage 
without stopping the continuous caster sequence. Heated slabs 7 exit the 
furnace 6 for subsequent processing in a tandem hot mill 12. The mill 12 
comprises a train of four high reversing mill stands. Preferably four such 
reversing mill stands are employed, designated 18, 20, 22 and 24 in the 
drawings. A coiler box 10, preferably an up coiler, is situated above the 
roller support table 9 intermediate the furnace 6 and the mill 12. A 
coiler 16, preferably a down coiler, for receiving finish strip is 
situated at the end of a run out table 14 downstream of the tandem hot 
mill 12. A conventional descaler 28 is also situated intermediate the 
furnace 6 and the tandem hot mill 12 for removing scale from the slabs 7 
prior to finish rolling. In addition, a crop shear (not shown) may also be 
situated between the coiler 10 and the tandem mill 12 for cropping the 
ends of the workpiece prior to the finish pass in the mill. 
In operation, a typical first pass is depicted in FIG. 1. A slab 7 enters 
the tandem hot mill 12 and is rolled therein in a first pass while 
utilizing the first three mill stands 18, 20 and 22 to reduce the slab in 
thickness while greatly extending the length thereof. During the first 
pass, the fourth mill stand 24 remains idle. The slab 7 is converted to a 
transfer bar 11 during the first pass, to transform an original slab 
length of for example 85 feet (26 meters) and a thickness of 4 inches (100 
mm) to a bar length of about 70 meters and a thickness of about 35-40 mm 
while rolling in a downstream direction along the run out table 14. The 
coiler 16, which is adapted to receive the finished hot strip material, is 
located a distance indicated D1 in FIG. 1 from the downstream end of the 
tandem hot mill 12. The distance D1 is greater than the anticipated length 
of the transfer bar 11 formed during the first pass from the mill 12 
which, in this case, would be in excess of 70 meters. 
The second pass is depicted in FIG. 2 wherein the transfer bar 11 is rolled 
in a reverse or upstream direction through the mill stands 18, 20 and 22 
for a further reduction in thickness and an increase in length to form an 
intermediate bar 13. The intermediate bar 13 is received in the coiler box 
10 as it exits the first stand 18 of the tandem hot mill 12. Typically, in 
the second pass, the transfer bar 11 is reduced to a thickness of between 
about 8 mm and 15 mm in thickness. 
In the third and final pass, depicted in FIG. 3, the intermediate bar 13 is 
withdrawn from the coiler box 10 and rolled in the downstream direction to 
a finish thickness through the mill stands 18, 20, 22 and 24. The finish 
strip material 15 is then fed into the down coiler 16 in a traditional 
manner. The fourth mill stand 24 is thus activated only during the last 
rolling pass to engage the strip 15 and impart the final reduction to 
size. In this manner, the work rolls of the final mill stand 24 stay in 
service longer than otherwise would be possible. During the final rolling 
pass depicted in FIG. 3, the intermediate bar 13 may be reduced to finish 
strip 15 having a thickness down to 1.5 mm. 
In order to process slabs 7 having a thickness greater than 4 inches, a 
one-way roughing mill stand 26 may be positioned at the exit side of the 
reheat furnace 6. In this manner, for example, a slab 7 having a thickness 
of 6 or 61/2 inches may be reduced in the one-way mill 26 to a thickness 
of 4 inches prior to entry into the first stand 18 of the tandem hot mill 
12. As seen in FIG. 1, the one-way rolling mill 26 is positioned upstream 
of the first mill stand 18 a distance indicated as D2 in the drawing. The 
dimension D2 is slightly in excess of the reduced slab length exiting the 
mill 26, which may be, for example, on the order of about 85 feet (26 
meters). 
In an alternate embodiment depicted schematically in FIG. 5, the elongated 
roller hearth furnace 6 shown in FIGS. 1-4 can be replaced with a walking 
beam furnace 6' which necessarily reduces the overall length of the 
installation. Such a walking beam furnace performs the same function as 
the elongated furnace 6, namely, to heat the pre-cut slabs to a proper 
rolling temperature prior to entry into the tandem hot mill 12. The 
walking beam furnace 6' may receive slabs from a slab caster 4, or from a 
second slab caster 30 situated on the same side, or from a caster 32 
located on an opposite side of the furnace 6'. 
The continuous strands of steel produced from the casters 4, 30 or 32 are 
cut to desired slab lengths by the torch cutters 8 and then subsequently 
fed into the walking beam furnace 6'. The heated slabs then exit the 
walking beam furnace 6' for rolling in the tandem hot mill 12 in the 
three-pass sequence as previously described. A shuttlecar may also be 
employed to connect the two lines of a two-strand caster for use in an 
insulating or roller furnace type. 
Thus, the process of the invention employs four reversing mill stands 
connected in tandem to reduce the slab thickness down to 1.5 mm. The 
process is completed in three rolling passes through the rolling mill 
train wherein the second pass is a reverse pass producing an intermediate 
bar which is temporarily coiled in the intermediate up coiler 10 so as to 
reduce the overall length of the mill. The last stand 24 of the mill train 
12 is preferably kept idle during the first two passes and used only 
during the last pass for the final strip touch-up. Thus, the present 
invention is advantageous due to the fewer number of mill stands installed 
as well as a lower installed motor horsepower. The coiler 10 may be fully 
or partially insulated or it may be heated if necessary to maintain the 
intermediate strip temperature. The run out table 14 may, likewise, be 
provided with a heavy-duty reversing segment immediately downstream from 
the tandem mill 12 to assist in the second reverse rolling pass. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. The presently preferred 
embodiments described herein are meant to be illustrative only and not 
limiting as to the scope of the invention which is to be given the full 
breadth of the appended claims and any and all equivalents thereof.