Field of Invention
The present invention relates to a process for manufacture of hot rolled coils
substantially free from rolled-in scale. More particularly, the invention relates to
a process, which reduces the level of rolled-in scale in hot rolled coils without
affecting the strength and other physical properties.
Background and Prior Art
Genesis of rolled-in scale in hot rolled coils has been discussed in several
literature. Various types of rolled-in scale, probable reasons of its occurrence
and measures of its control are described below:
Surface Defects in Hot Rolled Sheets, by Verlag Stahleisen, Germany, 1996
describes Sand scale composed of oxides and appearing as small sand dispersed
shape and comparative round. The scale is caused by surface defacement of the
work roll in the finishing train. Methods for controlling such sand scale
provided include increasing amount of coolant water; controlling bar
temperature after roughing mill and regulating reduction schedule in finishing
mill.
Surface Defects in Hot Rolled Sheets, by Verlag Stahleisen, Germany, 1996
describes various shape and depth of embedded scale, which appear during hot
rolling and remain as embedded form on the strip surface. Such scales are caused
by too low temperature at the entrance of finishing mill; absence of de-scaling or
poor de-scaling. Control measures include keeping reasonable temperature at the
entrance of finishing mill; keeping good condition of de-scaling nozzle and
changing to high pressure de-scaling.
Surface Defects in Hot Rolled Sheets, by Verlag Stahleisen, Germany, 1996 describes sharp
rolling of comet tail scale rolled into the surface. It is formed by spalling or pinholes on the
work roll surface. Control measures include increasing the roll coolant water and
keeping the good condition of the work coolant nozzles.
Surface Defects in Hot Rolled Sheets, by Verlag Stahleisen, Germany, 1996 describes the drag
scale, which is either of red colour spindle shaped or black colour and irregular shaped. Such
scaling is caused by scouring between bottom face of the strip and dust on the apron in the
finishing train. Measures to be taken for such scaling include keeping good pass
line through the mills and keeping good condition of each roll, apron and guide.
Surface Defects in Hot Rolled Sheets, by Verlag Stahleisen, Germany, 1996 describes linear type
scales, which are of linear form and embedded in the strip surface. They are caused by the
oxidation of the skin holes and blowholes in ingot. Such scaling may be controlled by
improving the ingot making and scarfing and controlling the temperature and the time
in the reheating furnace.
Surface Defects in Hot Rolled Sheets, by Verlag Stahleisen, Germany, 1996
describes Scaly scale (dot scale), which is of scaly form and embedded by the
work roll. This type of scaling is formed due to too high temperature at finishing
rolling. Control measures include keeping the proper temperature in the
finishing train.
Surface Defects in Hot Rolled Sheets, by Verlag Stahleisen, Germany, 1996 describes Red scale
is linear form or the band shaped and colour is red or brown. It has been shown that due to the
high Si%(=0.1 %)content roots of the scale ( Si-Fe-O) is very deep and remain after descaling.
For controlling such scaling the measures taken include keeping the temperature of the
reheating furnace high and changing to high pressure descaling (300/cm2).
US 5879465 teaches a method and apparatus for descaling hot rolled stainless steel strip which
comprises uncoiling the steel strip coil, removing the coil bias and cracking the surface scale in a
first scale breaking apparatus, flatening the strip by removing waves at the edges and buckle at
the center to a flatness, the tolerance of deviation depends upon the gauge of a strip, pre-cleaning
the strip, pickling the strip in a series of turbulent flow pickle tanks using a dilute acid, such as
hydrochloric acid and in presence of an acid accelerator, rinsing the pickled strip with water,
vigorously brushing the pickled strip for removing adherent alloying element oxide films not
removable by dilute acid pickling, drying the pickled strip, inspecting the strip, electrostatically
oiling the strip and recoiling the strip. Desirably, the strip is flattened in a two high temper mill,
which at the same time, elongates the strip, reduces its gauge and further cracks the surface
scale.
GB 2345492 A teaches a method for manufacturing a hot rolled galvanized steel sheet at a high
speed, with the pickling skipped, is disclosed, in which an intermediate rapid cooling is carried
out at a predetermined temperature so as to make the wustite component of the scales become
20% or more after a hot rolling, then a reducing heat treatment is carried out. so as to make the
wustite component of the scales become containing a predetermined amount of A1, thereby
realizing a superior coating adherence and a superior productivity. The hot rolled steel sheet is
cooled at a usual cooling rate, and is coiled. Then an intermediate rapid cooling is carried out on
the hot rolled steel sheet (thus coiled) to an intermediate rapid cooling temperature of 300-500°C
C so as to make a wustite component of scales become 20% or more. Then a reducing heat
treatment is carried out at a temperature of 550-700 C for 30-300 seconds under a 20% (or more)
hydrogen atmosphere. Then the hot rolled steel sheet (thus reduced) is dipped into a zinc bath
having an A1 concentration of 0.2-5.0 wt% whereby a superior coating adherence and a superior
productivity are realized.
It is known that in mills whose length is about 1 km, the distance from
Reheating Furnace #4 to the finishing mill is about 0.5 km. Due to long distance,
the heat loss from hot slabs due to radiation/convection is very high. It is for
this reason that in order to achieve a required temperature of 970-980° C before
the Finishing Mill, the drop-out temperature of slab (after leaving reheating
furnace) is very high (> 1300° C). This often leads to scaling.
Also a relatively much longer Run Out Table (ROT) for accommodating nearly 3
km long strip resulting from rolling of a 32 ton slab (maximum weight slab
rolled in the mill) adds to the problem.
Different grades of steel are rolled in the mill. Metallurgical properties in the
HR coil are maintained by controlling its finishing and coiling temperatures
besides its chemical composition. Final strip temperature after finishing is
maintained in the range 850-880° C and coiling in the range of 640-660° C. To
achieve 850-880° C strip temperature after finishing, furnaces were maintained
at 1350° C with slab drop out temperature of early finishing stands as 1280-1300°C
and bar temperature after roughing stand in the range of 1070-1100°C. All these lead to
development of scaling in the HR coil.
The roll bending along the barrel length due to strip resistance coupled with the
restraining of the roll ends lead to higher specific loads at the strip edges.
Consequently, the friction and wear of the works rolls is more at both the ends.
When the wear becomes sufficiently high, tertiary scale developed after the
finishing descaler (or between each finishing mill stand), may accumulate in the
roll cavities and after reaching certain limits, is removed from the work rolls
and imprinted on the strip surface, thus generating RIS. This is why the edges of
the strip arc more vulnerable to tertiary RIS and this is in accordance with the
present observations. The tertiary RIS is never removed from the strip as there is
no subsequent descaling operation. It continues to remain on the strip and
following pickling, the defect takes the form of faint surface depressions,
possibly containing residual scale particles on a rough undersurface, and
exhibiting gentle contours.
The bottom surface of the strip is more often at a higher temperature than the
top surface because the inter-pass cooling sprays in finishing train are directed
more effectively onto the top surface with resulting lower surface/ skin
temperature. As a result, the scale thickness is higher on the bottom surface of
the strip leading to a greater pick-up by the rolls and to thereby, more defects on
the bottom edges as in present case.
The higher incidence of the defects in the tail ends of the coils may be attributed
to mismatch of the opening and closing of pinch roll (finishing) hydraulic
descaler headers with the strip exit. In such case, the residual scale tends to get
rolled-in in the finishing train of the mill.
Thus there is a need to reduce the scaling in hot rolled steel coils and provide a
method for formation of hot rolled steel coils with minimum amount of scaling
which is suited for plants with large mill length.
The present inventors have now found that lowering finishing entry temperature
and reduction of hematite content on surface of coils leads to effective control
of scaling of hot rolled coils. The process is also cost effective manner.
Objects of Invention
Thus the main object of the present invention is to provide process of reducing
rolled in scales to produce hot rolled coils free from rolled in scale, without
affecting the desired overall quality of coils.
A further object is to provide a cost effective method for reducing scaling in hot
rolled coils.
Summary of Invention
Thus according to the main aspect of the present invention there is provided a
process for manufacture of hot rolled steel with reduced tertiary type rolled in
scales generated after the pinch roll descaler comprising
(i) maintaining slab drop out temperature in the reheating furnaces in the
range of 1200-1230°C; and
(ii) maintaining finishing entry temperature in range of 900-930°C so as to
achieve reduction of hematite to reduce such scaling on surface of
coils.
Detailed Description of Invention
Investigations conducted to characterize the type and the composition of scale
on hot rolled (HR) steel samples revealed that the same are tertiary type. The
scale was examined for its physical appearance, morphological features,
thickness, structure, composition and micro-hardness. Steel specimen with
rolled-in-scale (RIS) defects were also subjected to scanning electron
microscopy and X-ray diffraction analysis. It was found that the
scale is generated after the pinch roll descaler and gets "rolled-in"
in the early stands of finishing train. The general scale on the
surface of the coils is found to contain 15-30% hematite (Fe2O3);
enough to cause cavitations/ depressions on the work roll surface leading,
subsequently, to scale pick-up and transfer on to the strip.
It is further found that the high finishing entry temperatures of the transfer bar
(1000-1020 °C) are responsible for a greater hematite formation on the surface
of the strip. The deficiencies in pinch roll descaling (owing to inadequate spray
overlap, improper nozzle alignment and mismatch with strip entry and exit) and
the non-application of inter-stand cooling for thinner grades add to the problem.
Lower hematite content (5.6-7.5%) in scale can be achieved by lowering the
F#6-F#7 inter-stand temperatures of the transfer bar surface to the range of 915-
930 °C.
Further slab drop out temperature is reduced from 1300°C to 1220oC in the
reheating furnaces in order to achieve temperature of 920°C before the strip
enters finishing stands.
The rolling process speed is increased from 14 m/sec to 19 m/sec and number of
descalers was reduced from 6 to 3. Calculations showed that to obtain the
desired temperature of 920-950°C at the beginning of finishing mill, the
maximum drop out temperature should be 1200-1230°C , i.e., a reduction in drop
out temperature in the reheating furnace by 80°C.
Water curtain introduced in 5 descalers. Three descalers are now down, but
water curtain continues for removing secondary scale.
After Rough Stand 5, bar temperature is brought down from 1050 to 950°C to
reduce the formation of Fe2O3 , which is hardest of the three phases. Further,
because of this temperature reduction, the bar size was reduced from 38 to 35
mm so that the roll force in the finishing mill does not exceed the permissible
limit. The transfer bar temperatures at the exit to R#5 must be maintained in the
range of 1050-1070 °C so as to achieve a surface temperature between 930-950°C
in F#6-F#7 inter-stand region.
Top surface temperature of bar decreases rapidly as it is exposed to the
atmosphere, whereas bottom surface temperature does not fall that much. This
difference in temperature causes plastic deformation in the bar. To avoid this
problem, bottom surface was cooled to almost same temperature by using one
bottom header after Roughing stand #5.
Inter-stand cooling after F6, F7 and F8 was made operational to keep the skin
temperature of bar in the range 900-930°C so as to avoid further formation of
Fe2O3. Earlier it used to be about 980°C. It may be mentioned that no adjustment
was required in any of the rolling parameters on this account. Inter-stand cooling
has to be made effective and may be employed for all gauges to facilitate
elimination of R1S. This is required to keep the strip skin temperature below the
range corresponding to increased hematite formation (~ 930-950 "C).
Further modified pinch roll descaler (PRD) headers with high performance and
high impact 'Scalemaster' nozzles is incorporated for efficient removal of scales
from transfer bar surface before entry to finishing train.
The work roll surface wear and the recurrence of the RIS defects on hot rolled
coils may be observed after a change of work rolls in the finishing stands. This
helps in determining the workable rolling tonnage and rolling schedule length of
the coil in a particular campaign. Care should be taken not to exceed this
campaign length of rolled coil and new works rolls may be immediately replaced
each time.
The customary "painted slab" test may be conducted regularly to evaluate the
efficacy of the different hydraulic descaling stations.
The opening and closing of the finishing scale breaker headers are fine-tuned so
that the front and tail ends of the slab/ strip are completely descaled before
entering the finishing stands.
The invention is now described with reference to non-limiting accompanying
figures.
Brief Description Of Accompanying Figures
Figure 1 illustrates a flow chart of process of manufacture of hot rolled steel
with indications where improvement according to present invention is made.
Figure 2 illustrates equilibrium curves for haematite (Fe2O3), magnetite (Fe3O4)
and Wustite(FeO) as function of temperature.
Detailed Description Of Accompanying Figures
The flow chart (fig.l)shows the events in the process of HR steel manufacturing.
According to present invention the changes are brought about in the steps 5 zone
reheating furnace, water descaling, 5 roughing strands and 7 finishing strands
which are indiacted by (*). The slab drop out temperature is reduced from
1300°C to 1200-1230°C preferably 1220°C in the reheating furnaces in order to
achieve temperature of 920°C before the strip enters finishing stands.The
number of descalers is reduced from 6 to 3. However, water curtain is
introduced in 5 descalers. Three descalers are now down, but water curtain
continues for removing secondary scale.
After Rough Stand 5, bar temperature is brought down from 1050 to 950°C to
reduce the formation of Fe2O3 , which is hardest of the three phases. The transfer
bar temperatures at the exit to R#5 is maintained in the range of 1050-1070 °C
so as to achieve a surface temperature between 930-950°C in F#6-F#7 inter-
stand region.
Inter-stand cooling after F6, F7 and F8 is made operational to keep the skin
temperature of bar in the range 900-930 C so as to avoid further formation of
Fe2O3.
It is depicted in Fig. 2 that the scale consists of three components: FeO, Fe3O4
and Fe2O3. Of these three phases, FeO is the softest and Fe2O3 is the hardest,
which can make dents in the work roll surface. The temperature range in which
each one of these has high rate of formation is shown in Fig. 2. It may be noted
that the amount of hematite increases very rapidly above 930 °C. Of the three
oxide types, wustite (FeO) is the softest scale, while hematite (Fe2O3) is
significantly harder than the work rolls which have a Vickers hardness ranging
from 410 to 615. Temperature range corresponding to high rates of formation of
Fe2O3 is to be avoided.
Best mode of working the working the invention
The Table 1 below indicates the average production and down gradation due to
scaling before and after the use of the present invention. It is found that before
incorporation of improvement of the process of the present invention about 0.5%
of production had to be down graded or diverted. On introduction of the process
of the present invention such down gradation is nullified. This leads to cost
improvement in the process and adds to the benefit of the plant.

The X-ray diffraction spectra of all conventional steel samples, with RIS defects
or otherwise, showed significantly higher percentages of hematite (Fe2O3) in the
general scale ranging between 15-30% (Table-4) The average roughing exit
temperature of the transfer bar at R#5, as seen from Table-2 and Table-3, varies
from 1070-1100 °C. Around one meter of the front end of the strip has an entry
temperature, ahead of the finishing train, in the range of 1000-1020 °C and it
continues to be in the range favoring increased hematite formation upto finishing
stand no. 9 (F#9) in the 7-stand finishing train. Consequently, the roll wear is
more in the early stands of finishing mill, namely. F#6, F#7 and F#8 as the
temperatures here correspond to that of increased hematite formation. It has now
been found that the problem of increased hematite formation can be assuaged to
a large extent by intensification of inter-pass cooling so that the strip skin
temperature is maintained around 930 "C. Additionally, a lower rolling
temperature in the finishing mill is also advantageous. Additional cooling before
and after F#6 has been reported to have significantly lowered work roll wear and
improved roll life.

It has been found that the R#5 exit temperatures of the transfer bar were lowered
to 1050-1060 °C. Table-5 shows the scale layer composition of the coils, with
lower roughing exit and lower finishing entry temperatures, as determined by X-
ray diffraction. The F#6-F#7 inter-stand temperatures of the transfer bar surface
were maintained in the range 915-930 °C leading to significant reduction in
hematite content (5.6-7.5%) in the scale as against 15-30% for hot strip
processed with higher finishing entry temperatures of 1000-1020 °C.
Further a marked decrease in hematite content was also observed in experimental
coils with coiling temperatures less than 650 °C in spite of high finishing
temperatures. The hematite content in the scale, for these coils, ranged from 9-
12% as given in Table-6.

In the hot rolling of steel, it is essential to clean the scale from the steel surface
prior to entering the finishing rolling mill. Inadequate descaling can lead to poor
product quality and increased mill maintenance. Scale is removed by blasting
very large volume of high-pressure water on the steel surface. The descaling
process is totally mechanical and force that must be considered are
> Interface bond of the scale to the steel surface
> Force required to break this interface bond
> Criteria for the application of the force to remove the scale.
In past several manufacturers had developed new nozzle designs that can
increase the water impact on the scale surface with relatively lower water
pressure and flow rates. This is accomplished with a nozzle design providing a
knife edge, hard heating uniform jet. In the present invention a new nozzle
design is used that provides equivalent cleaning action at lower pressures and/or
water volume resulting in energy savings.
We Claim
1. A process for manufacture of hot rolled steel with reduced tertiary type
rolled in scales generated after the pinch roll descaler comprising:
(i) maintaining slab drop out temperature in the reheating furnaces
in the range of 1200-1230°C; and
(ii) maintaining finishing entry temperature in range of 900-930°C,
so as to achieve reduction of hematite to reduce such scaling on
surface of coils.
2. A process as claimed in claim 1 such that the hematite content is in the
range of 5 to 10 %, preferably 5.6 to 7.5 %.
3. A process as claimed in any preceding claim wherein slab drop out
temperature in the reheating furnaces is 1220°C.
4. A process as claimed in any preceding claim wherein number of descalers
is reduced from 6 to 3 while water curtain introduced in 5 descalers.
5. A process as claimed in any preceding claim wherein bottom surface
temperature of bar is cooled to almost same temperature as that of the top
surface by using one bottom header after Roughing stand #5.
6. A process as claimed in any preceding claim wherein the reduction in
hematite content achieved selectively by reducing after Rough Stand 5bar
temperature from 1050 to 950°C; avoiding temperature range of 1000°C to
1200°C corresponding to formation of haematite, by inter-stand cooling
after F6, F7 and F8 to keep the skin temperature of bar in the range 900-
930°C, preferably 915-930°C, more preferably 920°C
7. A process as claimed in any preceding claim wherein rolling speed is
increased from 12 - 14 m/sec to 17-20 m/sec, preferably 19 m/sec.
8. A process as claimed in any preceding claim wherein the bar size is
reduced from 38 to 35 mm so that the roll force in the finishing mill does
not exceed the permissible limit.
9. A process as claimed in any preceding claim wherein the transfer bar
temperatures at the exit to R#5 is maintained in the range of 1050-1070°C
so as to achieve a surface temperature between 930-950oC in F#6-F#7
inter-stand region.
10. A process as claimed in any preceding claim wherein modified fine tuned
pinch roll descaler (PRD) headers with high performance and high impact
'Scalemaster' nozzles is incorporated for efficient removal of scales from
transfer bar surface before entry to finishing train.

A process for manufacture of hot rolled steel with reduced tertiary type
rolled in scales generated after the pinch roll descaler comprising
maintaining slab drop out temperature in the reheating furnaces in the range
of 1200-1230°C and maintaining finishing entry temperature in range of 900-
930°C so as to achieve reduction of hematite to reduce such scaling on
surface of coils.