CA1078292A - Method of producing ferritic stainless steel for coinage - Google Patents
Method of producing ferritic stainless steel for coinageInfo
- Publication number
- CA1078292A CA1078292A CA265,093A CA265093A CA1078292A CA 1078292 A CA1078292 A CA 1078292A CA 265093 A CA265093 A CA 265093A CA 1078292 A CA1078292 A CA 1078292A
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- CA
- Canada
- Prior art keywords
- maximum
- stainless steel
- cold
- ferritic stainless
- thickness
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
ABSTRACT
A method of producing cold rolled ferritic stainless steel having a surface hardness and smoothness suitable for stamping into coins, which comprises providing a non-hardenable ferritic stainless steel casting having a maximum of 0.03% carbon by weight, hot rolling the casting to intermediate thickness, box annealing at a temperature of 815° to 845°C for at least 4 hours in a non-oxidizing atmoshere, abrading to remove up to 0.07 mm of the thickness, pickling in an acid solution, cold rolling to final thickness, and continuously annealing at 845° to 900°C in a hydregen atmosphere.
A method of producing cold rolled ferritic stainless steel having a surface hardness and smoothness suitable for stamping into coins, which comprises providing a non-hardenable ferritic stainless steel casting having a maximum of 0.03% carbon by weight, hot rolling the casting to intermediate thickness, box annealing at a temperature of 815° to 845°C for at least 4 hours in a non-oxidizing atmoshere, abrading to remove up to 0.07 mm of the thickness, pickling in an acid solution, cold rolling to final thickness, and continuously annealing at 845° to 900°C in a hydregen atmosphere.
Description
.
-`
:` -1078Z9Z ~` `
.;. ~.
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 ' ', ' .
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 . : , -` 1(378292 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 1~7829Z
-`
:` -1078Z9Z ~` `
.;. ~.
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 ' ', ' .
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 . : , -` 1(378292 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 1~7829Z
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.
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 :.
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 D - ~, oG~
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 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.
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.
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 :.
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 D - ~, oG~
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 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.
Claims (5)
1. 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 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 845°C, with a time at temperature of at least four hours, in a nonoxidizing 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 900°C in a hydrogen atmosphere.
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 845°C, with a time at temperature of at least four hours, in a nonoxidizing 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 900°C in a hydrogen atmosphere.
2. Method according to claim 1, including the further step of pickling said cold rolled and annealed strip in an aqueous solution containing 4% to 10% nitric acid and 1% to 2% hydrofluoric acid, by weight.
3. Method according to claim 1, wherein said steel 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, 0.25% maximum nickel, 0.15% to 0.25% aluminium, and balance iron except for incidental impurities.
to 0.75% silicon, 12% to 13% chromium, 0.25% maximum nickel, 0.15% to 0.25% aluminium, and balance iron except for incidental impurities.
4. Method according to any one of claims 1 - 3, wherein said cold rolling is conducted on a Sendzimir cold mill.
5. Method according to any one of claims 1 - 3, wherein the surface hardness of the cold rolled and annealed strip does not exceed Rockwell B 75, and the surface is 4.06 x 10-4mm maximum roughness.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA265,093A CA1078292A (en) | 1976-11-08 | 1976-11-08 | Method of producing ferritic stainless steel for coinage |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA265,093A CA1078292A (en) | 1976-11-08 | 1976-11-08 | Method of producing ferritic stainless steel for coinage |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1078292A true CA1078292A (en) | 1980-05-27 |
Family
ID=4107219
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA265,093A Expired CA1078292A (en) | 1976-11-08 | 1976-11-08 | Method of producing ferritic stainless steel for coinage |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1078292A (en) |
-
1976
- 1976-11-08 CA CA265,093A patent/CA1078292A/en not_active Expired
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