Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method of produoing
- cold rolled ferritic s~ainless steel strip having a low sur-
face hardness suitable for stamping into coins. More spe- ;
cifically, for coining operations a stainless steel hav mg
- 5 the following properties is needed: -
0.03~ maximum carbon
- 0.0025 inch (0.064 mm) thickness tolerance on 9S% of the
material -~
hardness of Rockwell B 75 maximum, or Vickers hardness with -
10 kg load (diamond indentor) of 140 maximum
good surface quality with minimum defects and 4.06xlO 4 mm
maximum roughness.
Conventional processing o non-hardenable ~
ferritic stainless steels includes melting in an electric
furnace or othe~ suitable melting vessel, casting into ingots
or continuous casting, grinding, and hot rolling to an inter- l
mediate thickness of e.g. 3.05 mm (0.12 inch). These `
steps are common to all conventional processes.' Thereafter,
several different subsequent routings have been practiced in
the prior ar~. In the so-called dull finish process the hot
rolled material is shot blasted or abraded to remove up
to 0.05 to 0.07 mm (0.002 to 0.003 inch) of the thickness,
pickled in an acid solution, cold rolled on a, tandem mill to
a final gauge which may range between 1.27 and 1.52 mm (0.05 to
0.06 inch), annealed in air at about 845C, and pickled in
a nitric acid-hydrofluoric acid solution.
Ferritic stainless steel processed in th~ above
manner exhibits a surface roughness exceeding the desired
4.06xlO 4mm maximum~for striking into coins. In an
attempt to improve the surface roughness, cold rolled ferritic
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stainless steel strip processed in the manner indicated
- above was subjected to a temper rolling or sKin pass of
0.5% reduction in thickness and tested. It was found
~`-~ that, àlthough the surface smoothness was improved, the -
hardness was too high and not uniform and resulted in ~ ;
excessive die wear and breakage during stamping or striking
of coins.
The conventional so-called bright finishing
(or "Rollbrite" finish) involves the steps of shot blasting
or abrading the hot rolled material to remove up to 0.05 to
O.07 mm of the thickness, acid pickling, flattening by
passage through a flexing and tensioning unit, coil grinding,
cold rolling in a Sendzimir cold mill to a final gauqe
which may range between 1.27 and 1.52 mm, and annealing in
a substantially pure hydrogen atmosphere at about ~45C.
This processing produced superior surface flatness
and satisfactory surface hardness for coining operations.
Bowever, the steps of flattening, coil grinding, and cold
rolling to final thickness in a Sendzimir mill made the
process prohibitive in cost. -
~ t is therefore an object of the present inventionto provide a method of pro~ucing cold rolled ferritic stainless
steel strip hav1ng a surface hardness suitable for stamping
into coins and good surface quality at a cost competitive with
is the least expensive prior art processin~ costs.
I According to the present invention, there is pro-
I vided a method of producing cold rolled, low carbon ferritic
stainless steel strip having a surface hardness and smoothness
suitable for stamping into coins, comprising the steps of
providing a non-hardenable ferritic stainless steel casting
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containing, in weight percent, 0.03% maximum carbon, 0.50%
maximum manganese, 0.040% maximum phosphorus, 0.020% ~-
maximum sulfur, 1.0% maximum silicon, 12% to 18% chromium,
0.50% maximum nickel, 0.1% to 0.3% aluminium, and balance
iron except for incidental impurities; hot rolling the casting
to intermediate thickness; box annealing the hot rolled steel
at a temperature of 815 to 845C, with a time at temperature of
at least four hours, in a non-oxidizing atmosphere; abrading
to remove up to 0.07 mm of the thickness; pickling in a nitric
acid-hydrofluoric acid solution; cold rolling to a final
thickness of 1.27 to 1.52 mm; and continuously annealing the
cold rolled strip at 845 to 900C in a hydrogen atmosphere.
For good corrosion resistance, it is preferred
to pickle the cold rolled and annealed strip in an aqueous
solution containing 4% to 10% nitric acid and 1% to 2%
hydrofluoric acid by weight.
It has been found that modification of standard
non-hardenable ferritic staînless steels by restriction of
the carbon content to a maximum of 0.03% instead of the usual
maximum ranging from 0.08% to 0.15%, in combination with an
anneal of the hot rolled steel at a temperature and for a
time sufficient to recrystallize the steel, results in a
surface hardness which does not exceed Rockwell B75 and
which is of satisfactory uniformity to permit coining oper-
ations without excessive die wear and breakage. It has also
been found that cold rolling followed by annealing in an
atmosphere of hydrogen, provides acceptable surface smoothness
without the necessity for a temper rolling or skin pass.
The final anneal after cold rolling in a hydrogen
atmosphere is a continuous anneal with a time ranging from
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2 to 3 minutes.
Cold rolling can be carried out either on a
Sendzimir cold mill or a tandem mill, the former being
preferred for improved surface smoothness and thickness
control. However, higher production rates are attainable
on a tandem mill.
Under some conditions cold rolling on a tandem
mill followed by annealing in a hydrogen atmosphere can
result in a surface having many minute indentations or
pits which have the appearance of small dark spots. It
has been found that such material is subject to severe
rus~ing in salt spray and high humidity environments.
It is believed that these small pits are chromium-
depleted areas which are associated with the formation
of oxide scale during hot rolling and which are not
completely removed by the pickling operation subsquent
to hot rolling. When this condition is observed in the r
cold rolled strip, a pickling step subsequent to the
final anneal has been found to restore the corrosion
resistance to that typical of ferritic stainless steels.
The pickling is carried out in an aqueous solution con-
taining 4% to 10% nitric acid and 1% to 2% hydrofluoric '
acid by weight, at a temperature of 55 to 65C, pre-
ferably at 60C. Other pickling steps previously des-
cribed are preferably also conducted under the same
conditions.
Generally the removal of the hot mill scale is
sufficient to avoid subsequent formation of black spots
in the cold rolled and annealed material if the hot
rolled material is subjected to abrading as by shot
blasting with iron grit or the like followed by an acid
pickle, resulting in removal of up to 0.07 mm of the
surfaces of the hot rolled steel.
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A preferred steel composition comprises, in
weight percent, 0.025% maximum carbon, 0..50% maximum
manganese, 0.030~ maximum phosphorus, 0.020% maximum
sulfur, 0.40% to 0.75% silicon, 12% to 13% chromium,
'5 0.25% maximum nickeL, 0.. 15% to 0.25% aluminium, and balan~e
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iron except for incidental impurities.'
The preferred box annealing temperature for .
the hot rolled steel'is 843Cj while the preferred
. continuous annealing temperature for the cold rolled
strip is 885C.
Ferritic stainless steel strip having the ~ r
.above preferred composition and processed in accordancè
with the method of the invention has been found'to meet
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JC~ the requirements of - O.G4 mm thickness tolerance on 95%
..15 of the'material, a Rockwell B s.urface hardness not exceeding
75, few surface defects and a 4.06x10 m maximum roughness.
. The.tandem mill which may be used in cold
rolling in the method of the invention comprises several
- stands in series of two-or four-high mills, thus permitting
cold reduction to final thicknesses of 1.27 to 1.52 mm
in a single'pass.
The Sendzimir or Z-mill is an arrangement wherein
each work roll is supported throughout its entire length.
by two first intermediate rolls which are in turn supported
by three second intermedi'ate ro~ls (of which the outer
ones are driven) which transfer the roll-separating forces
to a rigid, one-piece cast steel housing thro.ugh backing
- assemblies. The work rolls are driven by the four driven
rolls through friction contact with the first intermediate
rolls. Outer segmented rolls are provided which are actually
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rows of bearings on common shafts providing a caster-
like support action and permitting application of screw
pressure by rotating these shafts eccentrically. Such
a mill is capable of imparting a very smooth surface
within close thickness tolerances.
It will be understood that the use of ferritic
stainless steel for coinage is desirable because the
intrinsic value of the metal can be kept lower than the
face value of the coins, thus avoiding the tendency by
the public in some places to melt down the coins or to
hoard them. ;-
Since the cold reduced strip produced by the
method of this invention may have widths up to about
1.5 meters, a final optional step in the process
involves slitting the strip into widths convenient for
stamping coins (e.g~ 15 to 30 cm) prior to shipping.
