FIELD OF THE INVENTION
The present invention relates to a method of improving surface quality of electrolytic
tin plates. More particularly, the present invention is directed to providing electrolytic
tin plates free of recurrent surface defects in the form of wood-grain surface macro-
texture in reflowed electrolytic tin plate (ETP) coils. The method for producing
improved quality electrolytic tinplate with reduced surface defects like wood grain
comprised dozing and maintaining of levels of flux (phenol sulphonic acid) selectively
in the range of 1-2 g/l (as free acid) in drag-out fluxing tanks, maintaining a
consistent surface cleanliness level preferably <1000 mg/m2 in tin mill black plate
(TMBP) coils, regulating TMBP strip surface roughness within a narrow range of 0.4-
0.6 µm Ra, maintaining selective stannous (Sn2+) ion concentration within 20-25 g/l
and plating bath temperature ≥ 40 °C in Ferrostan® baths for achieving wider
covering power and brighter/ leveled tin electrodeposits with uniform reflowing
characteristics, favouring reduction in diversions and improvement in stampabiiity of
electrolytic tinplates (ETP) in electrolytic tinning lines (ETL) of steel mills.
BACKGROUND ART
The electrolytic tinning line (ETL) in Rourkela Steel Plant (RSP) of the applicants
produces about 15000 tonnes of electrolytic tinplate (ETP) annually with an overall
coating weight of 5.6 g/m2 for oil can manufacture. The thickness of the input cold-
rolled, annealed and skin-passed tin mill black plate (TMBP) sheet steel coils in the
line normally varies between 0.26-0.29 mm. Recurrent surface defects in form of
wood-grain surface macro-texture are commonly encountered in reflowed ETP coils,
which not only mar the surface appearance but also lead to substantial diversion of
coils as "seconds". The defect is so called as it closely resembles the grain of wood
and manifests as a pattern of alternate bands of diffuse (dull) and specular (lustrous)
reflectivity on reflowed tinplate. Usually, the defect spans the entire width of the
strip and may extend over several meters along the length of affected steel coils.
Although, wood grain does not impair the functional properties of tinplate like
solderability and corrosion resistance, nevertheless it adversely affects its
stampabiiity, a property of vital importance in canning industry. Conventional
electrolytic tinning process of tin mill black plate involves the stages of cleaning
(comprising alkali degreasing and acid pickling), electroplating, fluxing, flow-melting

and post-treatment sections involving cathodic dichromate (CDC) passivation and
oiling of tinplate. The tinplate exhibits a matte appearance after electroplating, which
is transformed to its characteristic lustrous finish through heat treatment procedure
called flow melting.
Electron microscopy and energy-dispersive X-ray analyses of defect-ridden ETP
specimens revealed that the pattern of alternate bands of diffuse and specular
reflectivity, associated with wood grain, were formed during the flow melting
operation in ETL. The defective surfaces were typically characterized by non-uniform
tin reflow and localized un-wetting of molten tin together with excessive Fe-Sn
alloying (FeSn2 formation) at coating-steel interfaces. Existing process and system
for electrolytic tinning of tin mill black plate were unable to identify the specific
process parameters/ variables and their role in development of surface defects like
wood grain and to implement desired control on such parameters to arrest the
recurrence of such defects.
Although some of the previous patents dealt with improving the electrolytic
deposition of tin on steel sheets on the following lines but none of them have
attempted to solve the problem of wood grain in tinning mills using the unique
methodology as herein described :
(a) Flux compositions for reflowing of tinplate;
(b) Tin plating electrolyte compositions and method of electroplating surfaces with
tin;
(c) Bath additives for improving brightness and appearance of tin electrodeposits;
(d) Reducing sludge in tin plating; etc.
WO/1998/013538 states about the improvement of the finish of electro-deposited
tinplate involving a new class of flux compounds, classified as dihydroxy or
polyhydroxy phenyl compounds containing one or more sulphonic acid or sulphonate
groups, which may be used as a flux on the tinplate prior to the reflow process and
ensure the production of a bright, reflective tin coating free from surface defects and
strongly inhibits woodgrain formation during reflow.

US 5427677 disclosed an improved process for reflowing tinplate by treating a
matte finish of tinplate with a flux, and reflowing the fluxed matte tinplate to produce
bright tinplate, wherein the flux is a naphthalenesulfonic compound, preferably
applied to the matte tinplate in an aqueous solution which may contain an acid. The
most preferred fluxes are 2-naphthol-6,8-disulfonic acid, l-naphthol-3,6-disulfonic
acid, and dipotassium 2-naphthol-6,8-disulfonate.
US 6409850 disclosed a method for improvement of the finish of electro-deposited
tinplate involving a new class of flux materials to be applied to matte tinplate prior to
reflow where the flux helps achieve a uniform bright tin finish. The invention
suggests a method for producing bright tinplate by immersing matte tinplate into an
aqueous solution of the fluxing compound, removing and drying the matte tinplate in
order to generate a tinplate coated with the fluxing agent and heating the coated
matte tinplate to a temperature above the melting point of the tin, but below that of
the steel, and quenching to generate a bright tinplate free from woodgrain.
The above mentioned prior patents and the state of the art would reveal attempts to
improve the surface quality of electroplated tin plates by involving new variety of
fluxes as well as involving treatment prior to reflow but there has been desired need
to further achieve uniformity in brightness and leveled tin coating while also
minimizing occurrence defects like wood grain, particularly to improve stampability
property of electrolytic tin plates.
There has been thus a need in the existing art of electrolytic tin plate manufacturing
to establish the genesis of wood grain macro-texture on the surface of
electrolytically-tinned sheets and implement remedial measures to alleviate defect
recurrence and increase standard acceptance level of tinned sheet product at
electrolytic tinning lines of steel plants.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to providing electrolytic tin
plates (ETP) produced from tin mill black plate (TMBP) free of surface defects like
wood grain and to identify the factors/process parameters responsible for such
defects and developing a method to minimize the occurrence of wood grain on ETP.

A further object of the present invention is directed to developing a method for
producing electrolytic tin plate avoiding occurrence of wood grain surface defects so
as to increase standard acceptance level of tinned sheet product and thereby
substantially reduce diversion of finished product into seconds.
A still further object of the present invention is directed to developing a method for
producing electrolytic tin plate avoiding occurrence of wood grain surface defects
such that the resulting tin product meets the requirement of stampability by the end
users of canning industry.
A still further object of the present invention is directed to providing electrolytic tin
plates with minimized wood grain on surface by controlling key operational
parameters (chemical) of ETL such as stannous ion (Sn2+) concentration of the
plating baths and the levels of phenol sulphonic acid flux (PSA as free acid) in drag-
out fluxing tanks.
A still further object of the present invention is directed to providing electrolytic tin
plates with minimized wood grain on surface by reducing the surface contaminants of
tin mill black plate (TMBP) before tinning and controlling its surface roughness to
desired level and to make it free of surface contaminants to eliminate the problems
of non-uniform reflow and de-wetting of molten tin (Sn) during reflow melting
process.
A still further object of the present invention is directed to providing electrolytic tin
plates with minimized wood grain and method for producing such tin plate coils
wherein work roll surface finish is selectively maintained at the skin pass mill with
desired level of surface roughness in order to achieve the TMBP strip surface
roughness closely controlled within a narrow range.
A still further object of the present invention is directed to providing electrolytic tin
plates with minimized wood grain and method for producing such tin plate coils
wherein wood grain due to non-uniform tin reflow and localized un-wetting of molten
tin together with excessive Fe-Sn alloying (FeSn2 formation) at coating-steel
interfaces is avoided.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to a process for the
manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed tin mill black plate_(TMBP) coils involving the step of reducing wood
grain in reflowed electrolytic tinplate comprising the steps of:
(i) selectively dozing and maintaining of levels of flux in drag-out fluxing
tanks;
(ii) maintaining a consistent surface cleanliness level of TMBP coils;
(iii) maintaining desired work roll roughness in skin pass mill;
(iv) restricting roll crown in skin pass mill;
(v) regulating TMBP strip surface roughness within defined ranges during skin
passing;
(vi) adhering to desired cycle-over-tonnage in skin pass mill for the TMBP
rolling; and
(vii) maintaining selective stannous (Sn2+) ion concentration alongwith desired
plating temperature in Ferrostan® plating baths such as to thereby
achieve a leveled, bright electrodeposit and an uniform reflowing of
molten tin in flow melting with reduced wood grain.
A further aspect of the present invention is directed to a process for the
manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed TMBP wherein the flux, phenol sulphonic acid_(PSA), is dozed at
1-2 g/i (as free acid) in drag-out fluxing tanks, which are located before flow
melting tower in electrolytic tinning line (ETL).

A still further aspect of the present invention is directed to a process for the
manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed TMBP wherein consistent surface cleanliness level of <1000 mg/m2
in tin mill black plate (TMBP) coils is maintained through intensification of alkali
degreasing and acid pickling processes in electrolytic tinning line (ETL).
A still further aspect of the present invention is directed to a process for the
manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed TMBP wherein the work roll roughness of 3-3.5 µm Ra is maintained
in Stand #1 with shot-peened surface finish and 0.4 µm Ra (maximum) in Stand #2
with ground/ polished finish in Skin Pass Mill (SPM) for achieving conducive mill finish
on input TMBP coils.
A still further aspect of the present invention is directed to a process for the
manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed TMBP wherein the roll crown is restricted to 0.04 and 0.02 urn,
respectively, in Stands # 1 and 2 of Skin Pass Mill.
According to yet another aspect of the present invention is directed to a process for
the manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed TMBP wherein the TMBP strip surface roughness is regulated within
a narrow range of 0.4-0.6 µm Ra and preferably, to below 0.5 µm Ra maximum.
A still further aspect of the present invention is directed to a process for the
manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed TMBP wherein cycle-over-tonnage of 200 tonnes is adhered to in
Skin Pass Mill for TMBP rolling.

A still further aspect of the present invention is directed to a process for the
manufacture of electrolytic tinplate (ETP) from cold-rolled, continuously annealed
and skin passed TMBP comprising maintaining stannous (Sn2+) ion concentration
between 20-25 g/l and plating temperature ≥ 40 °C in Ferrostan® plating baths for
achieving wider covering power and brighter/ leveled tin electrodeposits with uniform
reflowing characteristics.
Another aspect of the present invention is directed to an electrolytic tinplate (ETP)
from cold-rolled, continuously annealed and skin passed TMBP coils with reduced
wood grain obtained following the process as described herein above.
The objects and advantages of the present invention are described in greater details
with reference to the following accompanying non limiting illustrative drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the schematic illustration of the layout of conventional electrolytic tinning
line (ETL) showing the different stages involved.
Figure 2: is the schematic illustration of the process flow for manufacture of
electrolytic tinplate (ETP) from hot-rolled TMBP coils.
Figure 3: is the photo image of the reflowed electrolytic tinplate samples with and
without wood grain pattern of surface defects.
Figure 4: is the illustration of Auger electron micrographs and depth profile analysis
of ETP with wood grain, indicating that the defect is characterized by non-uniform
reflow and localized de-wetting of tin accompanied by excessive alloying at the Fe-Sn
interface.
Figure 5: is the illustration of scanning electron micrographs depicting comparative
appearance of electrolytic tinplates with and without wood grain surface defect.

Figure 6: is the bar chart for analysis of key operational parameters (chemical) of
ETL, showing the ranges of (a) Stannous ion (Sn2+) concentration of plating baths
(b) Phenol sulphonic acid flux (PSA as free acid) in drag-out fluxing tanks.
Figure 7: is the graphical presentation showing comparative analysis of surface
contaminants on TMBP coils from two different plants: (a) Range of oil contaminants
(after solvent degreasing) (b) Range of cumulative oil and unpickled oxide
contaminants (after solvent degreasing, alkali degreasing and acid pickling).
Figure 8: is the graphical plot of variation in surface roughness of TMBP coils
obtained from two different plants.
Figure 9: is the photo image showing the arrangement involving Hull cell used for
laboratory plating tests for measuring covering power and bright current density
range.
Figure 10: is the graphical presentation depicting the influence of plating variables on
widening of bright current density range or the covering power: (a) Effect of
stannous ion (Sn2+) concentration (b) Effect of plating bath temperature.
Figure 11: is the photo images showing visual appearance of tin electroplates from
laboratory plating experiments showing improvement in brightness and evenness of
tin electrodeposit with increase in stannous ion (Sn2+) concentration e.g. (a) 15 g/l
(b) 20 g/l and with increase in plating temperature e.g. (c) 30 °C (d) 40 °C
Figure 12: is the graphical presentation of the reduction in diversions of reflowed
electrolytic tinplate (ETP) on account of dull and dirty surface.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention is directed to providing a method for developing electrolytic tin
plates with minimized wood grain surface defects. The method of the invention is
having the prospects of being utilized/commercialized in tin mills and tin-plating lines
for improvement in surface quality of electrolytic tinplate, with uniformly reflowed
9

coating of tin with complete wetting of surface favouring reduction in diversions and
improvement in stampability of tinplates produced from such tin mills.
Reference is first invited to the accompanying Figure 1 that shows the schematic
layout of Electrolytic Tinning Line (ETL) at a tinning mill of the applicants comprising
cleaning, electroplating, fluxing, flow-melting and post-treatment sections involving
cathodic dichromate (CDC) passivation and oiling of tinplate. Accompanying Figure
2 shows the process flow for manufacture of electrolytic tinplate (ETP) from cold-
rolled, continuously annealed and skin passed TMBP coils in the ETL of Figure 1. In
conventional process, the tinplate exhibits a matte appearance after electroplating,
which is transformed to its characteristic lustrous finish through heat treatment
procedure called flow melting. During flow melting, the tinplate is heated by means
of resistance heating to the melting point of tin (~232 °C), whereby the electroplated
tin layer melts and reflows, before rapidly quenching in water to retain the
characteristic lustrous appearance associated with tinned canstock. It is experienced
in the existing process that the surface defect termed wood grain pattern develops
during the flow-melting operation, although pre-plating and plating variables may
predispose a coating to wood grain formation. The surface defect is characterized by
non-uniform reflow and localized de-wetting of molten tin during flow melting.
Accompanying Figure 3 shows surfaces of reflowed electrolytic tinplates with and
without wood grain pattern for comparison. The presence of wood grain pattern of
surface defects adversely affect the stampability of the tin plates which is essentially
required by the end users in can making industry.
In order to investigate the origin of wood grain surface defect in electrolytic tinplate
(ETP), defective sheet samples were initially subjected to Auger and scanning
electron microscopy concurrently with energy-dispersive analyses. The key
operational parameters (chemical) of ETL such as stannous ion (Sn2+) concentration
of the plating baths and the levels of phenol sulphonic acid flux (PSA as free acid) in
drag-out fluxing tanks were observed and analyzed over a 5-month period to identify
probable process deviations responsible for wood grain defects. The surface
characteristics of TMBP sheet coils were evaluated in terms of their average levels of
surface cleanliness (contaminants) and surface roughness in order to investigate the
problems of non-uniform reflow and de-wetting of molten tin (Sn) during reflow
melting process. The oil contaminants and tandem mill residues on coils were
assessed through solvent degreasing procedure (using organic solvents like ethanol,

iso propyl alcohol and acetone) while the levels of residual unpickled oxides on the
strips were evaluated by alkali degreasing at 80 °C in 25 g/l NaOH solution followed
by acid pickling in 5% v/v H2SO4 at room temperature. The process route adopted for
manufacture of TMBP coils was also studied at two different plants of the applicants
to identify specific inadequacies in coil processing prior to electrolytic tinning.
Laboratory electroplating experiments have also been conducted for assessment of
covering power of stannous ion plating bath with varying stannous ion (Sn2+)
concentrations (15, 20 and 25 g/l), plating bath temperatures (30, 40 and 50 °C)
and surface roughness of TMBP sheet panels (>0.5 µm and <0.5 µm Ra) using a
standard 267 ml Hull cell.
Electron microscopy and energy-dispersive X-ray analyses of defect-ridden ETP
specimens revealed that the pattern of alternate bands of diffuse and specular
reflectivity, associated with wood grain, are formed during the flow melting operation
in ETL. The defective surfaces are typically characterized by non-uniform tin reflow
and localized un-wetting of molten tin together with excessive Fe-Sn alloying (FeSn2
formation) at coating-steel interfaces. Accompanying Figure 4 shows the Auger
electron micrographs and depth profile analysis of ETP with wood grain showing that
the defect is a consequence of non-uniform reflow and localized de-wetting of tin
accompanied by excessive alloying at the Fe-Sn interface. Accompanying Figure 5
illustrates the comparative appearance of electrolytic tinplates with and without wood
grain surface defect as observed under a scanning electron microscope.
Analysis of operating variables over a 5-month period has been carried out in
electrolytic tinning line which revealed the shortcomings and inconsistencies of the
process variables leading to wood grain surface defects. It has been observed that
the ETL line was operated for 50-80% of the operational time at lower half of the
specified range for stannous ion (Sn2+) concentration, i.e. between 15-20 g/l as
against the set norm of 15-25 g/l. Accompanying Figure 6 gives the analysis of key
operational parameters (chemical) of electrolytic tinning line of one of the plants viz.
stannous ion (Sn2+) concentration of plating baths and phenol sulphonic acid flux
(PSA as free acid) in drag-out fluxing tanks over a typical 5-month period. It was
also observed that the line was operated at trace/ negligible levels of flux (PSA as
free acid) in drag-out fluxing tanks for 25-40% of the operational time. The
production norm for PSA (as free acid) in fluxing tanks was also not specified as a
quality target. Since flux dozing is imperative from the standpoint of improving tin

wettability, reducing tin oxidation and slagging contaminants from ETP surface
during flow melting process, necessary control measures have been implemented in
the shop to maintain PSA content in fluxing tanks between 1-2 g/l (as free acid) for
improving the reflowing characteristics of molten tin during flow melting.
The level of surface contaminants in TMBP coils was studied and measured in two
different plants of the applicants. Surface contaminants were found to range between
700-2200 mg/m2 in TMBP coils in a first plant as against 900-1600 mg/m2 in a
second plant. Investigations revealed that the surface contaminants in question were
not tandem cold mill oil/ lubricant residues but the residual oxide scales resulting
from under-pickling of TMBP sheet coils in sulphuric acid pickling being adopted at
the first plant. The incidences of wood grain were comparatively lower in TMBP coils
processed at the second plant due to pickling of hot-rolled TMBP. coils in hydrochloric
acid, which resulted in brighter pickling of steel surfaces with smoother surface
finish. Accompanying Figure 7 presents a comparative analysis of the level of
surface contaminants on TMBP coils from the two different plants of the applicants.
The higher level of surface contaminants (remnant oxides) on TMBP coils of the first
plant adversely affected the wettability and reflowing of molten tin during the flow
melting process. Adequate measures have been taken to improve the surface
cleanliness of TMBP coils through complete cleaning and recharging of alkali
degreasing and acid pickling tanks in ETL during an extended shut down of the line.
These steps brought down the level of surface contaminants to <1000 mg/m2 in
TMBP coils generated in first plant.
It has been further observed that the variation in surface roughness of TMBP coils led
to gross inconsistencies both in surface quality as well as tin reflowing behaviour
during flow melting. The inadequacies in roll dressing procedures leading to these
variations in surface roughness in TMBP sheet coils have been investigated and duly
addressed in Skin Pass Mill of the first plant. The surface roughness of TMBP coils is
found to vary widely between 0.31-0.97 µm with an average of 0.60 µm Ra.
Accompanying Figure 8 graphically depicts the variation in surface roughness of
TMBP coils at two different plants of the applicants. As a remedial measure, the work
roll roughness in Stand #1 of Skin Pass Mill has been maintained between 3-3.5 urn
Ra with shot-peened finish and an average roughness of 0.4 µm Ra maximum is
maintained for the work rolls in Stand #2 of the mill with ground/ polished finish.
The roll crown is also restricted to 0.04 and 0.02 urn, respectively, for Stands #1 and

2. The cycle-over-tonnage for TMBP rolling is restricted/ controlled to 200 tonnes in
Skin Pass Mill for achieving consistent coil surface quality. Through these set of
interventions, the TMBP strip surface roughness was closely regulated within a
narrow range (0.4-0.6 µm Ra).
Laboratory plating tests have been conducted for assessment of covering power
(bright current density range) of tin plating bath with varying stannous ion
concentrations (15, 20 and 25 g/l), plating bath temperatures (30, 40 and 50 °C)
and surface roughness of TMBP sheet panels (>0.5 µm and <0.5 µm Ra) using
standard Hull electroplating cell as illustrated in Figure 9. The covering power is a
measure of the ability of plating bath to achieve bright, leveled electrodeposits over
widest possible range of operational current densities. From laboratory experiments,
it was determined that stannous ion (Sn2+) concentrations between 20-25 g/l,
plating temperatures >40 °C and TMBP strip surface roughness <0.5 µm Ra were
conducive for obtaining wider bright current density ranges (1-4 A/dm2) and
brighter, leveled tin electrodeposits with uniform reflowing characteristics.
Accompanying Figure 10 graphically shows the widening of bright current density
range or the covering power with an increase in stannous ion (Sn2+) concentration
and plating bath temperature. Accompanying Figure 11 shows the visual
appearance of tin electroplates from laboratory plating experiments showing an
improvement in brightness and evenness of tin electrodeposit with increase in
stannous ion (Sn2+) concentration and plating temperature. Brighter electrodeposits
are more leveled and hence, reflow more uniformly under flow melting. Accordingly,
steps are taken in the plating baths in ETL to regulate the stannous ion (Sn2+)
concentrations within 20-25 g/l and plating temperatures >40 °C to achieve wider
bright current density ranges favouring bright and uniformly leveled tin
electrodeposit. Accompanying Figure 12 shows graphically the reduction in
diversions of reflowed electrolytic tinplate (ETP) on account of dull and dirty surface.
The diversions of ETP coils has been reduced substantially by around 46%.
The present invention is thus directed to providing a comprehensive methodology as
described hereinabove for minimizing the incidences of wood grain surface defect in
reflowed electrolytic tinplate that involves all of the following steps:

1. Dozing and maintaining of levels of flux (phenol sulphonic acid) at 1-2 g/l (as free
acid) in drag-out fluxing tanks, which are located before flow melting tower in
electrolytic tinning line (ETL).
2. Maintaining a consistent surface cleanliness level <1000 mg/m2 in tin mill black
plate (TMBP) coils through intensification of alkali degreasing and acid pickling
processes in electrolytic tinning line (ETL).
3. Maintaining work roll roughness of 3-3.5 µm Ra in Stand #1 with shot-peened
surface finish and 0.4 µm Ra (maximum) in Stand #2 with ground/ polished finish
in Skin Pass Mill (SPM) for achieving conducive mill finish on input TMBP coils.
4. Restricting roll crown to 0.04 and 0.02 µm, respectively, in Stands # 1 and 2 of
Skin Pass Mill.
5. Regulating TMBP strip surface roughness within a narrow range of 0.4-0.6 µm Ra
and preferably, to below 0.5 µm Ra maximum.
6. Adhering to cycle-over-tonnage of 200 tonnes in Skin Pass Mill for TMBP rolling.
7. Maintaining stannous (Sn2+) ion concentration between 20-25 g/l and plating
temperature ≥ 40 °C in Ferrostan® baths for achieving wider covering power and
brighter/ leveled tin electrodeposits with uniform reflowing characteristics.
It is thus possible by way of the present invention to providing a method for
producing electrolytic tin plate with minimized wood grain surface defects and thus
saving on diversion of substantial quantity of defective ETP coils into seconds and to
ensure desired stampability quality of the ETP produced to meet the requirement of
the canning industry as end users.

WE CLAIM:
1. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP coils in an Electrolytic Tinning Line with
Ferrostan® plating baths involving the step of reducing wood grain in reflowed
electrolytic tinplate comprising the steps of:
I. selectively dozing and maintaining of levels of flux in drag-out fluxing tanks;
II. maintaining a consistent surface cleanliness level of TMBP coils;
III. maintaining desired work roll roughness in skin pass mill;
IV. restricting roll crown in skin pass mill;
V. regulating TMBP strip surface roughness within defined ranges during skin
passing;
VI. adhering to desired cycle-over-tonnage in skin pass mill for the TMBP rolling;
and
VII. maintaining selective stannous (Sn2+) ion concentration alongwith desired
plating temperature in Ferrostan® plating baths such as to thereby achieve a
leveled, bright electrodeposit and an uniform reflowing of molten tin in flow
melting with reduced wood grain.
2. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP as claimed in claim 1 wherein the
flux ,phenol sulphonic acid, is dozed at 1-2 g/l (as free acid) in drag-out fluxing
tanks, which are located before flow melting tower in electrolytic tinning line
(ETL).
3. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP as claimed in anyone of claims 1 or

2 wherein consistent surface cleanliness level of <1000 mg/m2 in tin mill black
plate (TMBP) coils is maintained through intensification of alkali degreasing and
acid pickling processes in electrolytic tinning line (ETL).
4. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP as claimed in anyone of claims 1 to
3 wherein the work roll roughness of 3-3.5 µm Ra is maintained in Stand #1 with
shot-peened surface finish and 0.4 µm Ra (maximum) in Stand #2 with ground/
polished finish in Skin Pass Mill (SPM) for achieving conducive mill finish on input
TMBP coils.
5. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP as claimed in anyone of claims 1 to
4 wherein the roll crown is restricted to 0.04 and 0.02 µm, respectively, in
Stands # 1 and 2 of Skin Pass Mill.
6. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP as claimed in anyone of claims 1 to
5 wherein the TMBP strip surface roughness is regulated within a narrow range of
0.4-0.6 µm Ra and preferably, to below 0.5 µm Ra maximum.
7. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP as claimed in anyone of claims 1 to
6 wherein cycle-over-tonnage of 200 tonnes is adhered to in Skin Pass Mill for
TMBP rolling.

8. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP as claimed in anyone of claims 1 to
7 comprising maintaining stannous (Sn2+) ion concentration between 20-25 g/l
and plating temperature ≥ 40 °C in Ferrostan® plating baths for achieving wider
covering power and brighter/ leveled tin electrodeposits with uniform reflowing
characteristics.
9. An electrolytic tinplate (ETP) from cold-rolled, continuously annealed and skin
passed TMBP coils with reduced wood grain obtained following the process as
claimed in anyone of claims 1 to 8.
10. A process for the manufacture of electrolytic tinplate (ETP) from cold-rolled,
continuously annealed and skin passed TMBP coils involving the step of reducing
wood grain in reflowed electrolytic tinplate substantially as herein described and
illustrated with reference to the accompanying figures.

The present invention relates to a process for improving surface quality of reflowed
electrolytic tin plates. The process of the invention is directed to providing
electrolytic tin plates free of wood-grain surface macro-texture in reflowed
electrolytic tin plate (ETP) coils. The method for producing improved quality
electrolytic tinplate with reduced wood grain comprises dozing and maintaining of
levels of flux (phenol sulphonic acid) selectively in the range of 1-2 g/l (as free acid)
in drag-out fluxing tanks, maintaining a consistent surface cleanliness level
preferably <1000 mg/m2 in tin mill black plate (TMBP) coils, regulating TMBP strip
surface roughness within a narrow range of 0.4-0.6 µm Ra , maintaining selective
stannous (Sn2+) ion concentration of 20-25 g/l and plating bath temperature ≥ 40 °C
for achieving wider covering power and brighter/ leveled tin electrodeposits with
uniform reflowing characteristics, favouring reduction in diversions by about 46%
and improving stampability property of resulting ETP.