Apparatus for thermomechanically rolling hot strip product to a controlled microstructure

A hot strip mill having a final reducing stand and runout cooling means downstream of the reducing stand includes an incubator capable of coiling and decoiling the hot strip. The incubator is located intermediate the runout cooling means. In a preferred form the final reducing stand is a hot reversing mill. A second incubator and/or a temper mill and/or a slitter may be positioned downstream of the first incubator. The method of rolling includes isothermally treating the strip within a predetermined time and temperature range in the incubator prior to subsequent processing. The subsequent processing may include any one or more of the following: further deformation by cold rolling, temper rolling or cooling at a desired heat loss rate.

FIELD OF THE INVENTION 
Our invention relates generally to hot strip rolling methods and apparatus 
and more particularly to methods and apparatus for thermomechanically hot 
rolling strip steels or plates of various compositions to a controlled 
microstructure on a mill, which mill includes incubation means located 
intermediate the cooling means on the runout table associated with the hot 
strip or plate mill. 
DESCRIPTION OF THE PRIOR ART 
The metallurgical aspects of hot rolling steels have been well known for 
many years, particularly in respect of the standard carbon and low alloy 
grades. The last reduction on the final finishing stand is normally 
conducted above the upper critical temperature on virtually all hot mill 
products. This permits the product to pass through a phase transformation 
after all hot work is finished and produces a uniformly fine equiaxed 
ferritic grain throughout the product. This finishing temperature is on 
the order of 1550.degree. F. (843.degree. C.) and higher for low carbon 
steels. 
If the finishing temperature is lower and hot rolling is conducted on steel 
which is already partially transformed to ferrite, the deformed ferrite 
grains usually recrystallize and form patches of abnormally coarse grains 
during the self-anneal induced by coiling or piling at the usual 
temperatures of 1200.degree.-1350.degree. F. (649.degree.-732.degree. C.). 
For these low carbon steels the runout table following the last rolling 
stand is sufficiently long and equipped with enough quenching sprays to 
cool the product some 200.degree.-500.degree. F. (93.degree.-260.degree. 
C.) below the finishing temperature before the product is finally coiled 
or hot sheared where the self-annealing effect of a large mass takes 
place. 
It is further recognized that some five phenomena take place that 
collectively control the mechanical properties of the hot rolled carbon 
steel product. These five phenomena are the precipitation of the MnS or 
AlN or other additives in austenite during or subsequent to rolling but 
while the steel is in the austenite temperature range, recovery and 
recrystallization of the steel subsequent to deformation, phase 
transformation to the decomposition products of ferrite and carbide, 
carbide coarsening and interstitial precipitation of the carbon and/or 
nitrogen on cooling to a low temperature. 
After hot rolling the product is often reprocessed such as by normalizing, 
annealing or other heat treatment to achieve the metallurgical properties 
associated with a given microstructure as well as relieve or redistribute 
stress. Such a hot rolled product may also be temper rolled to achieve a 
desired flatness or surface condition. In addition, mill products 
processed after hot rolling such as cold rolled steel and tin plate are to 
a degree controlled by the metallurgy (microstructure) of the hot rolled 
band from which the other products are produced. For example the hot band 
grain size is a factor in establishing the final grain size even after 
deformation and recrystallization from tandem reducing and annealing 
respectively. 
Heretofore, the semi-continuous hot strip mills as well as the so-called 
mini-mills which utilize hot reversing stands provide continuous runout 
cooling by means of water sprays positioned above and/or below the runout 
table extending from the last rolling stand of the hot strip mill to the 
downcoilers where the material is coiled or to the hot shears where a 
sheet product is produced. This runout table cooling is the means by which 
the hot band is cooled so as to minimize grain growth, carbide coarsening 
or other metallurgical phenomena which occur when the hot band is coiled 
or sheared and stacked in sheets and self-annealing occurs due to the 
substantial mass of the product produced. 
The various heat treatments and temper rollings which are utilized to 
achieve desired properties and shape occur subsequent to the hot mill 
processing per se. For example, where a certain heat treatment is called 
for, the coiled or stacked sheet product is placed in the appropriate heat 
treating facility, heated to the desired temperature and thereafter held 
to accomplish the desired microstructure or stress relief. 
In-line heat treatment has been employed with bar and rod stock. However, 
the surface to volume ratio of such a product vis-a-vis a hot band 
presents different types of problems and the objective with rod and bar 
stock is generally to obtain differential properties as opposed to the 
uniformity required of most hot strip products. Finally, in today's 
market, processing flexibility and the desired microstructure are more 
important than the sheer productivity capability of the mill. Existing hot 
strip facilities are primarily geared for productivity and therefore are 
not compatible with today's market demands. 
SUMMARY OF THE INVENTION 
Our invention recognizes the demands of today's market and provides 
flexibility and quality within the hot strip mill itself. At the same time 
it aids the productivity of the overall steel making operation by 
eliminating certain subsequent processing steps and units and 
consolidating them into the hot rolling process. We are able to operate 
within narrow target time and temperature ranges. In so doing we are able 
to provide a hot strip product with a controlled and reproducible 
microstructure. 
Our invention further provides a new product development tool because of 
its ease of operation and substantial flexibility. 
The phase transformations encountered in the rolling and treating of steels 
are known and are shown by the available phase diagrams and the kinetics 
are predictable from the appropriate TTT diagrams and thus a desired 
microstructure can be obtained. In addition, recovery and recrystalization 
kinetics are known for many materials. Heretofore hot mills were 
drastically limited in that regard because of the inflexibility of the 
tail end of the hot rolling process. 
This flexibility is made possible by providing an incubator capable of 
coiling and decoiling the hot strip and locating that incubator 
intermediate the runout cooling means so as to define a first cooling 
means upstream of the incubator and a second cooling means downstream of 
the incubator. A second or additional incubator(s) may be used in-line. 
The incubator may include heating means or atmosphere input means to give 
further flexibility to the hot rolling process. In addition, a temper mill 
and/or a slitter may be positioned in-line at a point where the strip is 
sufficiently cooled to permit proper processing. 
The method of rolling generally includes causing the strip to leave the 
final reducing stand at a temperature above the upper critical A.sub.3, 
cooling the strip to a temperature below the A.sub.3 by first cooling 
means, coiling the strip in the incubator to maintain temperature and 
cause nucleation and growth of the ferrite particles in the austenite, 
thereafter decoiling the strip out of the incubator and cooling it rapidly 
to minimize grain growth and carbide coarsening. Where the temper mill is 
employed the strip may then be temper rolled after being cooled to the 
appropriate temperature. By maintaining temperature it is meant that we 
seek to approach an isothermal condition, although in practice there is a 
temperature decay with time which we seek to minimize. 
A further means of processing hot strip includes utilizing a hot reversing 
mill as the final mill and reducing the band through the penultimate pass 
at a temperature above the A.sub.3 and thereafter cooling the strip and 
coiling the strip in the incubator to maintain temperature. Thereafter the 
strip is passed through the hot reversing mill for its final pass prior to 
further treatment utilizing the cooling means and the incubator. The 
process may also include utilizing a second incubator to control the 
precipitation phenomenon. 
Our method and apparatus find particular application with the hot reversing 
mill which in conjunction with the incubator provides a thermomechanical 
means for achieving a hot rolled band with a controlled microstructure. It 
also has particular application to steel and its alloys although other 
metals having similar transformation characteristics may be processed on 
our apparatus and by our method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The standard semi-continuous hot strip mill is illustrated in FIG. 1. The 
slab heating is provided by means of three reheat furnaces FC1, FC2 and 
FC3. Immediately adjacent the reheat furnaces is a scale breaker SB and 
downstream of the scale breaker SB is the roughing train made up of four 
roughing mills R1, R2, R3 and R4. The slab which has now been reduced to a 
transfer bar proceeds down a motor-driven roll table T through a flying 
crop shear CS where the ends of the transfer bar are cropped. The 
finishing train in the illustrated example comprises five finishing stands 
F1, F2, F3, F4 and F5 where the transfer bar is reduced continuously into 
the desired strip thickness. The finishing train is run in synchronization 
by a speed cone which controls all five finishing stands. 
The strip exits F5 at a desired finishing temperature normally on the order 
of 1550.degree. F. (843.degree. C.) or higher with the specific finishing 
temperature being dependent on the type of steel. The strip then passes 
along the runout table RO where it is cooled by means of a plurality of 
water sprays WS. After being cooled to the appropriate temperature by the 
water sprays WS the strip is coiled on one of two downcoilers C1 and C2. 
It will be recognized that the schematic of FIG. 1 is just one of many 
types of semi-continuous hot strip mills in existence today. It will also 
be recognized that the water sprays on the runout table may be any of 
several known types which provide cooling to one or both sides of the 
strip. 
The semi-continuous hot strip mill of FIG. 1 can be modified to include our 
incubator as shown in FIG. 2. The incubator I is positioned along the 
runout table RO and intermediate the water sprays so as to define a first 
set of water sprays WS1 upstream of the incubator and a second set of 
water sprays WS2 downstream of the incubator. The incubator can be located 
above or below the pass line. The incubator I must have the capability of 
coiling the strip from the final finishing stand and thereafter decoiling 
the strip in the opposite direction toward the downcoilers. A number of 
such coilers are known and the details of the coiler do not form a part of 
this invention. The incubator may also include heating means to provide 
external heat to the product within the incubator and may also include an 
atmosphere control such as a carbon dioxide enriched atmosphere to cause 
surface decarburization, a hydrocarbon enriched atmosphere to cause 
surface carburization or an inert atmosphere so as to prevent scaling or 
accomplish other purposes well known in the art. The details of the heat 
or atmosphere input into the incubator do not form a part of this 
invention. 
The optimum use of an incubator is in conjunction with a mini-mill which 
includes or is comprised of a hot reversing stand as shown in FIG. 3. With 
a hot reversing mill, it is possible to have deformation, temperature 
reduction and delay times independent of subsequent or prior processing. 
This is not as easily accomplished on semi-continuous mills where a single 
speed cone controls the rolling of a plurality of mills. This finds 
particular applicability where it is desired to eliminate subsequent 
reheating and heat treatment and where heating and rolling are used in 
conjunction such as in the controlled rolling of pipeline grade steels 
where a heat treatment (in this case a temperature drop) is employed prior 
to the final deformation. The hot mill processing line includes a 
reheating furnace FC1 and a four-high reversing mill HR having a standard 
coiler furnace C3 upstream of the mill and a similar coiler furnace C4 
downstream of the mill. Again the incubator I is positioned along the 
runout table RO intermediate the cooling means so as to provide a first 
set of water sprays WS1 upstream of the incubator I and a second set of 
water sprays WS2 downstream of the incubator I. 
Since it is now possible to hold the strip in the incubator I the strip may 
be sufficiently cooled through the downstream cooling means WS2 so that a 
temper mill and/or a slitter may be included in line as part of the hot 
strip mill. Such an arrangement is illustrated in FIG. 4 where a temper 
mill TM and a slitter S are positioned downstream of the second cooling 
means WS2 and the strip after being rolled, cooled, incubated and water 
cooled a second time passes through the temper mill at temperatures on the 
order of 300.degree. F. where it is appropriately flattened, thereafter 
slit and then coiled on a coiler C5. 
Multiple in-line incubators can be used with a hot reversing mill to 
achieve even more control over the metallurgical and physical qualities of 
the product of the hot strip mill. Such arrangements are shown 
schematically in FIGS. 5 and 6. The hot strip mill of FIG. 5 is similar to 
that of FIG. 3 except that an additional incubator I2 is positioned 
downstream of the second cooling means WS2 and a third cooling means WS3 
is positioned downstream of the second incubator I2 and upstream of the 
final downcoiler C1. The arrangement of FIG. 5 may be further modified 
through the addition of a temper mill TM and coiler C5 positioned 
downstream of the third set of water sprays WS3 as shown in FIG. 6. A 
slitter could also be incorporated into the mill. 
Our invention is also applicable to plate mills where a reversing stand is 
employed. This is shown in FIG. 9 where a large slab exits the furnace FC1 
and is reduced on the hot reversing mill PM between the coiler furnaces C3 
and C4. The coil is then cooled by water sprays WS1 and thereafter coiled 
in the incubator I. While in the incubator, the appropriate heat treatment 
is carried out. Multiple incubators may be employed. The coil is 
thereafter decoiled and passed along the runout table RO where it is air 
cooled (AC) prior to being sheared by in-line shear PS. The plates are 
then stacked or otherwise transferred to cooling tables as is conventional 
in the art. The advantage is that large slabs such as 30 tons or more can 
be processed into plates and the conventional small pattern slabs can be 
eliminated. In addition this increases yields to on the order of 96% from 
the conventionally obtained 86% yields. Subsequent heat treatment can be 
eliminated in many instances. 
The use of our incubator gives tremendous flexibility and microstructure 
control in the hot rolling of a hot band. Heretofore, the microstructure 
of the hot band was controllable only through composition, finishing 
temperature and coiling temperature. We are now able to control (a) phase, 
nucleation and transformation, (b) recovery and recrystallization, and (c) 
precipitation through the use of the in-line incubator or incubators. 
The standard iron carbon phase diagram, FIG. 7 defines the thermodynamic 
feasibility of effecting a phase transformation. The solubility limits are 
essential in depicting the temperature phase relationships for a given 
composition. The rate of approach to these equilibrium phases is defined 
by the total sum of all the kinetic factors which are embodied in the 
standard TTT diagrams of which the diagram of FIG. 8 for a low carbon 
steel is representative. The TTT diagrams specify the temperature and 
transformation products that can be realized at some period of time. We 
are able to literally walk the product through the TTT diagram. In 
addition, by prenucleating ferrite, it is possible to shift the TTT curves 
and achieve shorter times for transformation. 
The morphology of transformation products that develops is based on solid 
state diffusion of alloy components, the nature of the nucleus of the new 
phase, the rate of nucleation and the resultant large scale growth effects 
that are the consequences of simultaneous nucleation processes. The 
conditions under which nucleation are effected during the incubation 
period will have a major effect on the overall morphology. 
In general, in crossing a phase boundary transformation does not begin 
immediately, but requires a finite time before it is detectable. This time 
interval is called the incubation period and represents the time necessary 
to form stable visible nuclei. The speed at which the reaction occurs 
varies with temperature. At low temperatures diffusion rates are very slow 
and the rate of reaction is controlled by the rate at which atoms migrate. 
At temperatures just below the solvus line the solution is only slightly 
supersaturated and the free energy decrease resulting from precipitation 
is very small. Accordingly, the nucleation rate is very slow and the 
transformation rate is controlled by the rate at which nuclei can form. 
The high diffusion rates that exist at these temperatures can do little if 
nuclei do not form. At intermediate temperatures the overall rate 
increases to a maximum and the times are short. A combination of these 
effects results in the usual transformation kinetics as illustrated in the 
TTT diagram of FIG. 8. 
The phenomenon that occurs while the product is in the incubator is related 
to forming the size and distribution of nuclei. When this time is complete 
the phenomena that follow are largely growth (diffusion) controlled at a 
given temperature. In other words, the nature of the final reaction 
product can be controlled by changing events during the incubation period. 
For this reason the utilization of one or more incubators provides 
virtually a limitless number of process controls to achieve a totally 
controlled microstructure. 
The overall apparatus and process of our invention is based on the 
recognition that grain refinement is a major parameter to control in order 
to effect major changes in mechanical properties. The substance of this 
control is exercised by creating metallurgical processing of the steel 
that will yield a fine, uniform grain size. During the final stages of the 
deformation, for example, on the hot reversing mill the finish pass is 
effected under a controlled temperature to result in deformation just 
above the A.sub.3 (typically, although there are steels where just below 
the A.sub.3 becomes an improtant pass temperature) resulting in a 
metallurgical condition of deformation bands splitting up the austenitic 
grains. Controlling the subsequent holding temperature permits 
recrystallization based on the time chosen and the kinetics of the 
material. Having achieved the desired microstructure, it can be maintained 
by an immediate reduction of the strip temperature through a controlled 
and specified cooling rate on the runout table on the way to the 
incubator. The final temperature achieved during this runout cooling is 
chosen such that the steel goes into the incubator at a temperature 
required by the TTT diagrams. This may be in the range of normal coiling 
temperature if a ferrite-pearlite microstructure is desired, it may be 
several hundred degrees below that if an acicular bainitic structure is to 
be achieved, or it may be between the A.sub.1 and A.sub.3 if prenucleation 
of ferrite is desired. 
As previously stated, the incubator can be utilized to control (a) phase, 
nucleation and transformation, (b) recovery and recrystallization and (c) 
preciptation. Additionally, there is the opportunity to inter critical 
anneal in the incubator. 
Further runout cooling after the incubator accomplishes a controlled 
reduction of remaining interstitials (such as carbon and nitrogen in 
excess of solubility limits) negating subsequent strain aging phenomena if 
applicable to the steel. 
Of course in low carbon materials that have a high MS temperature the 
incubator step can be bypassed entirely. With an appropriate hold in the 
coiler furnace of the hot reversing mill just above the A.sub.3 the steel 
can be quenched directly on the runout table to ambient temperatures 
producing martensite, where it can be further processed such as by temper 
rolling. In addition, the incubator can be used for simple delay purposes 
to coordinate with a subsequent operation independent of the speed of the 
prior operation. For example, it would now be possible to utilize in-line 
slitting and/or temper rolling whereas these processes have heretofore 
been independent of the hot strip mill. 
A key concept in these various processes is to complete recrystallization 
prior to effecting TTT reaction products. In addition the concept of grain 
splitting through deformation makes its unnecessary to cool steel to room 
temperature to produce a martensitic grain splitting followed by reheating 
as is usually done commercially. Thus, we have a fully continuous process 
to produce final metallurgical properties direct from the hot strip mill. 
The classification found in the Table 1 presents a number of materials by 
major alloy component along with the temperature and time at the shortest 
reaction route of the TTT diagram. This gives an indication of the length 
of hold times necessary for a wide variety of alloy steels and implies the 
relative feasibility of effecting transformations in times compatible with 
normal mill practices. Generally increasing carbon or alloy content 
decreases transformation rates. Increasing the austenite grain size has 
the same type of effect, but increasing the in-homogenity of austenite 
will increase the transformation rate. The steels listed in Table 1 are 
exemplary of the many steels which are amenable to processing by our 
method and apparatus. 
TABLE 1 
______________________________________ 
STANDARD STEELS AND ALLOYS 
Reaction Kinetics 
From TTT Diagrams 
Type AISI Designation 
T, .degree.F. 
T, .degree.C. 
t, Sec. 
______________________________________ 
Plain Carbon 
1035 1100 4 
Mn 1340 1100 60 
Mo 4027 900 15 
Mo 4037 900 70 
Mo 4047 900 70 
Cr--Mo 4130* 1225 180 
800 100 
Cr--Mo 4140* 1200 275 
700 200 
Cr--Mo 4150* 1200 450 
700 800 
Ni--Cr--Mo 
4340 800 15 
Ni--Cr--Mo 
8620* 1200 1000 
825 60 
Ni--Mo 4615 900 140 
Ni--Mo 4815 825 80 
______________________________________ 
*TTT curves include two curve noses 
As a class of materials, the alloys of the Table 1 have a high degree of 
hardening ability and have moderate reaction times at standard coiling 
temperatures. This permits the effective use of undissolved carbides in 
the austenite which act as nuclei to speed up the start of transformation 
and at the same time retard grain growth by pinning grain boundaries. The 
reaction times of the above materials are controllable by pre-nucleating 
in the incubator at temperatures between the A.sub.1 and A.sub.3. 
Other metals having similar transformation characteristics can also be 
utilized with our invention. For example, titanium goes through a Beta 
phase transformation where prenucleation takes place and thus titanium 
could be rolled utilizing our invention. The following are examples of 
several types of processing that can be carried out with steels on our hot 
strip mill utilizing at least one incubator positioned intermediate a 
cooling means on the runout table. 
EXAMPLE 1 
An improved hot rolled strip of standard low carbon steel is finish rolled 
at 1550.degree. F. (843.degree. C.) using standard drafting practice. The 
initial cooling is carried out by the first set of water sprays and at a 
speed to drop the temperature of the strip to 1100.degree. F. (593.degree. 
C.) at which time it is coiled in the incubator and held for five seconds. 
Thereafter it is uncoiled and further cooling brings the temperature to 
850.degree. F. (454.degree. C.) prior to final downcoiling. Normally such 
a product is coiled in the range of 1350.degree. F. (704.degree. C.) at 
which temperature sulfide precipitation is effected to pin the grain 
boundaries. Thereafter as the coil is self-annealed the carbides tend to 
coarsen after phase transformation is completed permitting some degree of 
grain growth. With the above-improved process, the cooling to 1100.degree. 
F. (593.degree. C.) retains a fine recrystallized grain size and permits 
phase transformation to occur independently of precipitation of sulfide 
and negates any opportunity for grain growth due to carbide coarsening. 
Subsequent cooling to a coiling temperature of 850.degree. F. (454.degree. 
C.) allows interstitials to precipitate on further slow cooling in the 
coil. This process provides a hot rolled strip with improved mechanical 
properties and a lighter scale because of the low temperatures involved. 
EXAMPLE 2 
For a drawing quality low carbon steel the hot band is cooled to near the 
A.sub.3 but not into the two phase region. Thereafter a final heavy draft 
is taken on a hot reversing mill to promote recrystallization of nuclei. 
The coil is then run into the incubator for on the order of two minutes to 
complete recrystallization. Thereafter runout cooling occurs at 25.degree. 
C. (77.degree. F.) per second and further runout cooling occurs at a few 
degrees per second. Finally a temper pass at 300.degree. F. (149.degree. 
C.) is carried out to create dislocations for precipitation. 
EXAMPLE 3 
For a normalized steel the strip is processed through hot rolling in the 
usual manner except that prior to the last pass on a hot reversing mill 
the strip is payed out onto the runout table to cool to 50.degree. F. 
(10.degree. C.) above the A.sub.3 at which temperature it is put into the 
incubator to equalize temperature. Thereafter a final reduction on the 
order of 30% is taken on the hot reversing mill to create deformation 
bands within the recrystallized austenite. Thereafter the strip is put 
back into the incubator furnace or into a second incubator furnace for 
about 100 seconds at greater than 1600.degree. F. (871.degree. C.). The 
strip is thereafter payed out onto the runout table and cooled to 
1100.degree. F. (593.degree. C.) at a rate of 50.degree. F. (10.degree. 
C.) per second. Again the strip is fed into the incubator for about 60 
seconds at about 1100.degree. F. (593.degree. C.). The strip is then 
cooled to 800.degree. F. (427.degree. C.) on the runout table prior to 
final coiling. 
EXAMPLE 4 
A martensitic steel can be produced by processing at a normal deformation 
schedule on a four-high hot reversing mill. Prior to the last pass the 
strip is sent onto the runout table and cooled to 50.degree. F. 
(10.degree. C.) above the A.sub.3 where it is put into the incubator to 
equalize temperature. The final pass produces a 30% reduction sufficient 
to create deformation bands within the recrystallized austenite. The strip 
is placed into the hot reversing coil furnace for a momentary hold and 
thereafter it is payed out along the runout table and fast cooled to 
300.degree. F. (149.degree. C.). It is then passed through the temper 
mill. 
EXAMPLE 5 
Dual phase steels are characterized by their lower yield strength, high 
work hardening rate and improved elongation over conventional steels. A 
typical composition would include 0.1 carbon, 0.4 silicon and 1.5 
manganese. The cooling rate from the inter critical annealing temperature 
has been found to be an important process parameter. Loss of ductility 
occurs when the cooling exceeds 36.degree. F. (2.2.degree. C.) per second 
from the inter critical annealing temperature. This is believed to be due 
to the suppression of carbide precipitation that occurs. Using our hot 
strip mill the normal hot rolling sequence is followed. The strip is 
cooled to the desired inter critical temperature with runout cooling and 
thereafter it is placed in the incubator at 1380.degree. F. (749.degree. 
C.) for two minutes. Thereafter additional runout cooling is provided at 
36.degree. F. (2.2.degree. C.) per second maximum cooling rate until the 
temperature reaches about 570.degree. F. (299.degree. C.). Alternatively 
this process could be optimized by putting the coil into a second 
incubator when the temperature on the runout table reaches 800.degree. F. 
(427.degree. C.) where it is known that carbide precipitation will occur. 
The function of a second incubator is to effect nearly complete removal of 
carbon from solution to produce a material that is soft and ductile. 
EXAMPLE 6 
High strength low alloy steels may be processed the same as the normalized 
steel of Example 3 except that a longer incubation period at 1100.degree. 
F. (593.degree. C.) is required. Times on the order of 180 seconds are 
required and thereafter standard cooling may be employed. 
It can be seen that our invention provides an almost limitless number of 
processing techniques to provide a controlled microstructure for a 
thermomechanically rolled hot strip product. Since entire subsequent 
processing steps and apparatus can be eliminated, lengthened runout tables 
and increased cooling means are economically feasible.