STEEL COMPOSITIONS, METHODS OF MANUFACTURING AND USES IN THE PRODUCTION OF RIMMELL PERCUSSION CARTRIDGES

Abstract

The present invention relates generally to steel compositions, methods for manufacturing the compositions, and the use of the compositions to produce rimfire cartridges. The steel compositions for use in rimfire cartridges are processed through cold rolling and annealing steps to create suitable physical properties.

Description

STEEL COMPOSITIONS, METHODS OF MANUFACTURING AND USES IN THE PRODUCTION OF RIMMELL PERCUSSION CARTRIDGES CROSS REFERENCE TO RELATED APPLICATIONS This patent application claims the benefit of United States Provisional Patent Application No. 62 / 092,359, entitled “Steel Compositions, Methods of Manufacture and Uses in Producing Annular Percussion Cartridges”, filed on December 16, 2014, the contents of which are incorporated herein by reference. FIELD OF INVENTION The invention relates in general to steel compositions, methods for manufacturing the compositions, and the uses of the compositions for producing cartridges for rimfire ammunition. BACKGROUND OF THE INVENTION Description of the related technique In general, rimfire cartridges are strong enough to withstand the pressures created by the ignition of a propellant, while remaining elastic enough to allow extraction from the chamber or barrel of a firing device after firing. Bronze has traditionally been used for this type of ammunition. Its physical properties allow for the manufacture of rimfire cartridge cases that meet the requirements for both strength and elasticity. Bronze is corrosion-resistant, formable, and quite elastic. Thus, the use of bronze results in few, if any, problems when the cartridge is extracted from the firearm after firing. Bronze is work-hardened to a degree that provides adequate strength to withstand the explosive force of the gunpowder charge with minimal failure of the cartridge sidewalls.It is relatively soft and therefore can be formed with minimal tool wear during manufacturing. This has made bronze a preferred material for manufacturing rimfire ammunition cartridges. However, a disadvantage of bronze is its cost; bronze is typically expensive and its price is known to fluctuate significantly. To develop a less expensive alternative metal, steel has been considered as a replacement for bronze. Advantageously, stress corrosion cracking and reaction with primers and gunpowder are not problems associated with using steel. However, a disadvantage is that steel does not have the same elastic recovery as bronze. As a result, extraction problems arise when removing steel cartridges from the chamber or barrel of the firearm after firing. For example, with the use of low-carbon steels, such as C1008 / 1010, extraction problems can be severe because the elasticity of low-carbon steel is much lower compared to bronze. Figure 1 is a graph of the stress-strain curves for bronze and steel, and shows the elastic strain for steel 1, the elastic strain for bronze 2, the total strain to failure for steel 3, the total strain to failure for bronze 4, the yield strength for bronze 5, the tensile strength for bronze 6, the yield strength for steel 7, the tensile strength for steel 8, and the slope 9. The Young's modulus, for example, the elastic recovery, for bronze and steel can be determined based on the slope of each of the stress-strain curves in the elastic region. The slope of the curve for bronze is half that of steel, as shown in Figure 1. The Young's modulus for bronze is approximately 1.03 x 10⁶ bar (15 x 10⁶ psi) while the modulus for steel is approximately 1.99 x 10⁶ to 2.06 x 10⁶ bar (29 x 10⁶ to 30 x 10⁶ psi).In Figure 1, the slope 9 is calculated as the stress divided by the strain in the elastic region. The elastic strain of steel 1 and the elastic stress of bronze 2 are shown in Figure 1. The elastic recovery for bronze is twice that of steel. Thus, bronze has almost twice the elasticity of steel for the equivalent steel level. As a result, a bronze cartridge, when fired, will expand in diameter due to internal pressure and essentially seal the chamber's internal diameter. After firing, the bronze cartridge will then "contract" in diameter so that its diameter is smaller than the chamber's internal diameter, and therefore, the cartridge can be easily removed from the chamber. Figure 2 is a schematic showing a portion of a firing device, including a cartridge head 11 and a cartridge sidewall 12, positioned within a chamber 13 of a barrel 14 of the firing device, and an extractor 15 for use in extracting the cartridge 11, 12 from the chamber 13 after firing the firing device. Figure 2 also includes a bolt 10, a firing pin 16, and a sidewall 17 that seals the chamber. Because the elasticity and springback of steel are significantly less than those of bronze, the diameter of a typical low-carbon steel cartridge will expand to seal the chamber upon firing. After firing, the diameter of the low-carbon steel cartridge will contract less—for example, only half as much as bronze—because (as shown in Figure 1) bronze has about half the elasticity of steel. The amount by which the diameter of the steel cartridge contracts may not be sufficient to allow the cartridge to be easily extracted from the chamber. As a result, after firing, the cartridge may become stuck in the barrel chamber of the firing device. Additionally, as shown in Figure 2, one or more sidewall separations (which is exaggerated) may occur with low-carbon steel because the material, even after forming and work-hardening, will not be strong or ductile enough to withstand the internal explosion experienced by the cartridge after firing. Without intending to be bound by any particular theory, it is believed that for steel to recover elastically to the same degree as bronze, the steel must have approximately twice the yield strength of bronze in the cartridge's extraction sidewall (after work-hardening during forming). However, it is quite likely that yield strength values ​​less than twice that of steel (in the cartridge sidewalls) could be sufficient to allow acceptable extraction after firing. Alternatively, higher carbon steels could be used to increase cartridge strength and overcome the aforementioned problems; however, there are anticipated issues with tooling and wear, and the steel would likely be too hard for the firing pin to deform the cartridge rim. Generally, rimfire cartridges have rims that can be deformed by the firing pin as a mechanism to ignite the powder charge contained within the cartridge case. Heat-treated, pre-formed steel cartridge cases can reduce tool wear and increase strength. U.S. Patent No. 2,373,921 to Snell and U.S. Patent No. 2,698,268 to Lyon describe a method for forming steel cartridge cases that require a heat-treating or annealing step after the case is formed. However, heat treatments on a batch of small parts, similar to rimfire ammunition cases, do not produce uniform results on all parts. Unlike Snell or Lyon, the steel rimfire cartridge of the present invention requires no further treatments after the case is formed.Furthermore, neither Snell nor Lyon contemplate the use of steel casings formed by their methods for use in rimfire ammunition, but instead apply the invention for the production of centerfire ammunition. Thus, there is a need in the art to design and develop a metal or metal alloy for use in the manufacture of rimfire ammunition cartridges that is a replacement for the typical bronze material known in the art. BRIEF DESCRIPTION OF THE INVENTION The present invention relates generally to steel compositions and methods for processing steel compositions to produce rimfire cartridges with steel cases. In one aspect, the invention provides a steel composition for rimfire ammunition cartridges. The composition includes from approximately 0.03 to approximately 0.18 percent by weight of carbon, from approximately 0.15 to approximately 1.60 percent by weight of silicon, from approximately 0.60 to approximately 2.50 percent by weight of manganese, from more than 0 to approximately 0.025 percent by weight of phosphorus, from more than 0 to approximately 0.025 percent by weight of sulfur, and from approximately 0.20 to approximately 0.08 percent by weight of aluminum, based on the total weight percent of the composition. The composition may also include one or more metallic elements selected from the group consisting of cobalt, niobium, chromium, copper, molybdenum, nickel, titanium, vanadium, zirconium, and mixtures and alloys thereof. One or more of the metallic elements present in the composition may typically constitute no more than approximately 0.22 percent by weight, based on the total weight of the composition. In certain forms, the composition may include from approximately 0.05 to approximately 0.13 percent by weight of carbon, from approximately 0.15 to approximately 0.50 percent by weight of silicon, from approximately 0.70 to approximately 2.50 percent by weight of manganese, approximately 0.025 percent by weight of phosphorus, approximately 0.025 percent of sulfur, from approximately 0.20 to approximately 0.08 percent by weight of aluminum, and less than approximately 0.22 percent by weight of one or more metallic elements, based on the total weight of the composition. In certain other forms, the composition may include from approximately 0.16 to approximately 0.18 percent by weight of carbon, from approximately 1.25 to approximately 1.55 percent by weight of silicon, from approximately 1.9 to approximately 2.1 percent by weight of manganese, approximately 0.02 percent by weight of phosphorus, approximately 0.02 percent of sulfur, from approximately 0.025 to approximately 0.055 percent by weight of aluminum, less than approximately 0.06 percent by weight of copper, less than approximately 0.04 percent by weight of nickel, less than approximately 0.06 percent by weight of chromium, and less than approximately 0.02 percent by weight of molybdenum based on the total weight of the composition. In certain other modalities, the composition may include approximately 0.126 to approximately 0.154 percent by weight of carbon, from approximately 0.395 to approximately 0.605 percent by weight of silicon, from approximately 1.75 to approximately 1.95 percent by weight of manganese, approximately 0.02 percent by weight of phosphorus, approximately 0.005 percent of sulfur, from approximately 0.02 to approximately 0.06 percent by weight of aluminum, less than approximately 0.06 percent by weight of copper, less than approximately 0.04 percent by weight of nickel, less than approximately 0.06 percent by weight of chromium, and less than approximately 0.02 percent by weight of molybdenum, based on the total weight of the composition. In another aspect, the invention provides a method for processing a steel composition for a rimfire cartridge.The method includes obtaining a steel composition having an original thickness, cold rolling the steel composition to reduce the original thickness by at least 70%, to produce a cold-rolled steel composition having an intermediate thickness, first annealing and then cooling the steel composition with an intermediate thickness to produce an annealed intermediate steel composition, cold rolling the annealed intermediate steel composition to a thickness that is reduced by approximately 20% to approximately 35% of the intermediate thickness of the intermediate steel composition to produce a steel composition having a final thickness, second annealing and then cooling the steel composition having a final thickness to produce a final annealed steel composition having a final thickness, and continuous cladding of the final annealed steel material having a final thickness. In some configurations, the first and second annealing steps are conducted as a batch process. In other configurations, the first and second annealing steps are conducted as a continuous process. In certain processes, the continuous plating step can be performed before the second cold rolling step. This continuous plating step can be in addition to, or instead of, the continuous plating step performed after the second annealing and cooling step. The continuous plating can include zinc, bronze, or combinations and alloys of these metals. The resulting steel composition may have an original thickness of approximately 0.22 cm (0.090 in). Furthermore, the resulting steel composition may be in a selected form of hot-rolled sheet, hot-rolled sheet that is pickled and oiled, and dual-phase cold-rolled sheet. The resulting steel composition may be at least partially reduced, such that the reduction in the first cold-rolling step can be modified or eliminated. In some processes, the resulting steel composition is an intermediate tri-rolled composition. In others, the resulting steel composition is a dual-phase cold-rolled composition. In these processes, an initial annealing and quenching step is performed before the first cold rolling. The method for forming the rimfire cartridge can be selected from a cup, pull and head process, and a progressive die and head process. In another aspect, the invention provides a method for processing a steel composition to form a rimfire cartridge. The method includes obtaining a steel composition of original thickness, cold-rolling the steel composition to produce a steel composition of final thickness, annealing and subsequently cooling the steel composition of final thickness to produce a final annealed steel composition of final thickness, and continuously plating the final annealed steel composition of final thickness. The rimfire cartridge may include a case made of the steel composition mentioned above, having a first end and a second end, a ring formed over the first end of the case, a projectile pressed into the second end of the steel case, a priming compound contained within the ring, and a propellant contained within the case. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing a stress / strain curve for each of bronze and steel. Figure 2 is a schematic showing a cartridge for rimfire ammunition inside a chamber of a firing device. Figures 3A, 3B, and 3C generally show a batch process according to the invention used to treat or process steel and form it into a rimfire cartridge case. Figure 4 is a flow diagram for a continuous process route #1 and a continuous process route #2 according to the invention used to treat or process steel and form it into an annular percussion cartridge case. DETAILED DESCRIPTION OF THE INVENTION Rimfire cartridges are generally known in the art and are typically composed of bronze and manufactured using conventional methods employed for cartridge processing. These methods include the cup-and-cap method, or alternatively, the progressive die-and-cap method. The invention provides steel compositions as replacements for bronze in the manufacture of rimfire cartridges, particularly for use in .22 caliber firing devices. The invention also provides methods for processing and treating, for example, by annealing, the steel compositions to produce rimfire cartridges exhibiting one or more of the following properties: • relatively soft in the ring so that the firing pin will deform the material enough to ignite the primer with the firing of a firing device; • strain hardened to a level that reaches twice that of typical bronze (which may be somewhat less in the batch annealed product) to achieve sufficient springback to avoid problems associated with cartridge extraction from the chamber in the barrel of the firing device; • that is sufficiently formable to produce the cartridge; and • sufficiently strong and ductile to reduce failures of the cartridge side walls, for example, separations, with firing of the firing device. The steel compositions produced according to the invention may vary and may depend on the particular steelmaker and the amount of alloying components used. In certain embodiments, the steel composition includes from approximately 0.03 to approximately 0.18 percent by weight of carbon, from approximately 0.15 to approximately 1.60 percent by weight of silicon, from approximately 0.60 to approximately 2.50 percent by weight of manganese, from more than 0 to approximately 0.025 percent by weight of phosphorus, from more than 0 to approximately 0.025 percent by weight of sulfur, and from approximately 0.02 to approximately 0.08 percent by weight of aluminum, based on the total weight percent of the composition.In addition, the compositions may include one or more metallic elements selected from the group consisting of cobalt, niobium, chromium, copper, molybdenum, nickel, titanium, vanadium, zirconium, and mixtures and alloys thereof. In certain embodiments, when one or more of these metallic elements are present in the compositions, they typically constitute no more than approximately 0.22 percent by weight, based on the total weight of the composition. In certain forms, the composition may include from approximately 0.05 to approximately 0.13 percent by weight of carbon, from approximately 0.15 to approximately 0.50 percent by weight of silicon, from approximately 0.70 to approximately 2.50 percent by weight of manganese, approximately 0.025 percent by weight of phosphorus, approximately 0.025 percent of sulfur, from approximately 0.20 to approximately 0.08 percent by weight of aluminum, and less than approximately 0.22 percent by weight of one or more of the metallic elements, based on the total weight of the composition. In certain other forms, the composition may include from approximately 0.16 to approximately 0.18 percent by weight of carbon, from approximately 1.25 to approximately 1.55 percent by weight of silicon, from approximately 1.9 to approximately 2.1 percent by weight of manganese, approximately 0.02 percent by weight of phosphorus, approximately 0.02 percent of sulfur, from approximately 0.025 to approximately 0.055 percent by weight of aluminum, less than approximately 0.06 percent by weight of copper, less than approximately 0.04 percent by weight of nickel, less than approximately 0.06 percent by weight of chromium, and less than approximately 0.02 percent by weight of molybdenum, based on the total weight of the composition. In certain other forms, the composition may include from approximately 0.126 to approximately 0.154 percent by weight of carbon, from approximately 0.395 to approximately 0.605 percent by weight of silicon, from approximately 1.75 to approximately 1.95 percent by weight of manganese, approximately 0.02 percent by weight of phosphorus, approximately 0.005 percent of sulfur, from approximately 0.02 to approximately 0.06 percent by weight of aluminum, less than approximately 0.06 percent by weight of copper, less than approximately 0.04 percent by weight of nickel, less than approximately 0.06 percent by weight of chromium, and less than approximately 0.02 percent by weight of molybdenum, based on the total weight of the composition. According to the invention, steel compositions undergo various processing steps to provide steel materials suitable for use in forming rimfire ammunition cartridges. The steel compositions can be obtained or received from a producer in various forms known in the art. For example, the steel composition can be received in a hot-rolled condition, either as a black strip (with intact oxide; to be pickled or deoxidized, for example, by a consumer) or in a pickled and oiled condition. The steel in the hot-rolled condition is processed on a hot strip mill, which can result in the following initial mechanical properties, for example: approximately 80 KSI minimum yield strength, approximately 95 KSI minimum tensile strength, and approximately 10% minimum elongation in 5.08 cm (2”).Alternatively, steel as received from the producer may have approximately 110 KSI yield strength, approximately 113 KSI tensile strength, and approximately 16% elongation in 5.08 cm (2”). These steels are commonly used in the hot-rolled condition, and it is desirable that the mechanical properties be as received by the end user, e.g., the customer. In certain scenarios, there may be no guaranteed or quoted properties from the hot-rolled steel producer because the end user will be generating the final properties through their own processing, including rolling and cold annealing. Typically, the end user can specify general hot-rolled steel parameters related to finishing and cooling temperatures, such as "high finishing temperatures" and "low cooling temperatures." In other scenarios, the steel may be received from the producer in an intermediate gauge compared to hot-rolled steel. In these scenarios, the steel may be supplied as either cold-rolled (hard tempered), regular annealed, or dual-phase annealed (or another higher-strength structure).Intermediate gauge steel can be used to shorten processing steps (designed for adjustment in the downstream processing scheme) or to utilize intermediate properties. Dual-phase intermediate steels can have various mechanical properties. In certain forms, dual-phase steel may have approximately 130.8 KSI yield strength, approximately 165.9 KSI tensile strength, and approximately 10.6% elongation in 5.08 cm (2"), or approximately 154 KSI yield strength, approximately 182 KSI tensile strength, and approximately 9.9% elongation in 5.08 cm (2"). In either case, the steel, as received from the producer, is then processed into a finished annealed product for use in the production of rimfire ammunition cartridges. Steel processing includes annealing, which can be carried out using either a batch or continuous process. The steel as received (from a producer) and to be processed may have the typical initial mechanical properties, as mentioned earlier, and is pre-treated by removing oxide and cutting to a width that allows for further processing. Figures 3A, 3B, and 3C are schematics showing the typical steps carried out in the batch annealing process of steel composition. The process steps in Figures 3A and 3B assume that the steel composition as received from the producer has not been processed. That is, the steel composition is in the form of an unprocessed hot sheet or a hot sheet in a pickled and oiled condition.However, it is understood that the steel composition as received from the producer may have been processed to a certain degree and, therefore, may be in the form of a partially processed sheet. In this situation, where the received sheet has undergone partial processing by the producer, it may be appropriate to modify or eliminate a step in the processes as shown in Figures 3A and 3B. For example, the processes shown in Figures 3A and 3B begin with pickling (deoxidizing) 20 of the sheet. If the sheet is received in a pickled condition, this initial step can be skipped or eliminated. Furthermore, the next step identified in Figures 3A and 3B is the cold rolling 22 of the steel composition to an intermediate thickness, which represents a reduction of approximately 70% of the initial thickness of the steel composition.If the steel composition received from the producer, for example, the starting steel composition, has already been partially processed, a 70% cold rolling reduction may not be necessary. In this way, this initial cold rolling step 22 can be modified or even eliminated to adapt the properties and degree of processing of the received steel sheet. In one embodiment, the received steel sheet may be in the form of a dual-phase cold-rolled sheet that has been partially processed, such that the cold rolling step can be performed to reduce the thickness by at least approximately 30% (instead of approximately 70% as mentioned in Figures 3A and 3B). Furthermore, with regard to a received steel sheet that is in the form of a dual phase, it may be appropriate to perform an annealing, for example, batch annealing, before the initial cold rolling step.This situation involves a dual-phase cold sheet received as shown in Figure 3C, which will be described later in this document. According to Figures 3A and 3B, after the initial cold rolling step 22, the steel composition having the intermediate thickness is subjected to an initial or first batch annealing 24 and a subsequent cooling 26. In Figure 3A, these batch annealing and cooling steps 24, 26 are followed by continuous plating 28. However, in Figure 3B, the continuous plating 28 is not carried out until the end of the process, for example, after a second or final batch annealing 32 and a cooling process 34. It is contemplated and understood that the continuous plating 28 can be carried out according to either of the processes mentioned in Figures 3A and 3B, and, additionally, the continuous plating 28 can be carried out according to both of the processes in Figures 3A and 3B.That is, continuous plating 28 can be carried out after the initial or first batch annealing 24 and the subsequent cooling process 26, and after the final or second batch annealing 32 and the subsequent cooling process 34. Continuous plating 28 includes applying or depositing a coating composition to form a layer or coating on the plating. The coating may include elemental zinc or a zinc alloy, elemental bronze or a bronze alloy, or another protective coating. After the first batch annealing and cooling and 24, 26 (and optionally plating 28), the steel composition is subsequently subjected to a second cold rolling process 30 which reduces the thickness by an additional 20-35% to a final thickness, followed by a second or final batch annealing 32 and cooling 34, to produce the annular percussion cartridges 36 according to conventional techniques. In certain methods, batch annealing involves heating the steel to approximately 260°C (500°F) for about 0.5 hours. The temperature is then increased to approximately 676.7°C (1,250°F) for 8.5 hours, and subsequently increased to approximately 704.4°C (1,300°F) for 1.5 hours, at which point it is held for approximately 6.0 hours. The steel is then cooled to room temperature and clad. After continuous cladding, the steel is further processed by cold rolling to a final thickness, providing approximately 20 to 35% additional reduction.Without claiming to be bound by any particular theory, it has been found that limiting further reduction to a variation between approximately 20% and approximately 35% produces steel with different physical properties that are typical for the steel grade. The steel at its final thickness is batch-annealed, cooled, and then formed into rimfire cartridges. The batch process described in Figure 3C shows the typical processing steps for using intermediate-thickness steel 40, either in the cold-rolled or dual-phase annealed condition. Figure 3C includes an initial or first batch annealing and cooling 42, followed by cold rolling 44 to an intermediate thickness (approximately -70% of the initial thickness), and then another or second batch annealing and cooling 46. Subsequently, a second cold rolling 48 is conducted to provide a final thickness (approximately a further 20-35% reduction), followed by another final batch annealing and cooling 50. As shown in Figure 3C, continuous plating 52 of the final annealed steel composition is carried out such that the steel composition is suitable for use in forming an annular percussion cartridge 54.In Figure 3C, continuous plating is shown as the final step before forming the cartridges. However, as in Figure 3A, one option is to perform continuous plating 52 after the second batch annealing 46, then cold rolling 48 to a final thickness, and subsequently the final batch annealing 50. The processes used to form rimfire cartridges may include conventional apparatus and methods known in the art, such as, but not limited to, cup, extraction and head processes, and progressive die and head processes. It should be noted that for the processes shown in Figures 3A, 3B, and 3C, the processing can also include rolling directly to a gauge without intermediate annealing. In this case, one consideration is the propensity for ear stretching. If rolling directly from a hot strip, a high degree of reduction (e.g., in the range of approximately 85 to approximately 88%) can help minimize ear stretching, depending on the chemistry. This alternative or option can also be applied to Figure 4, which will be described later. Figure 4 is a schematic showing the steps that can be conducted in a continuous processing, for example, annealing, of a steel composition. Figure 4 identifies process route #1 and process route #2. As shown in Figure 4, process route #1 involves starting with hot-rolled high-strength steel that is approximately 0.22 cm (0.090 in) thick. If the hot sheet is not received in a pickled condition, pickling or deoxidizing 60 is conducted to produce pickled hot sheet 62 (as identified in Figure 4 as the initial step). The pickled hot sheet 62 is cold-rolled 64 to a finished gauge or thickness, and then continuous annealing 66 and subsequent rapid cooling 68 are performed, followed by continuous plating 70 with zinc, bronze, or another protective coating.The resulting processed steel composition is then used to produce the formed 72 rimfire cartridges using conventional techniques. Additionally, as shown in Figure 4, process route #2 includes starting with a pickled, hot-rolled 60,62 high-strength steel approximately 0.22 cm (0.090 in) thick, as shown in process route #1. Furthermore, process route #2 includes cold rolling 64A to an intermediate thickness, followed by intermediate continuous annealing 66 and rapid quenching 68, followed by further cold rolling 74 to a final thickness providing approximately 20–35% reduction, then continuous annealing 76 and rapid quenching 78, followed by continuous plating 70 with zinc, bronze, or another protective coating. The resulting processed steel composition is then used to produce the formed 72 rimfire cartridges using conventional techniques.In process route #1, the continuous annealing temperature is typically approximately 968.3°C (1775°F). Subsequent cooling is for approximately 12 minutes to about room temperature before recoil at the exit end of the continuous annealing furnace. For the steel composition described in paragraph

[0033] herein, the continuous annealed structure produced by process route #1 (as shown in Figure 4) may consist of very fine-grained ferrite (approximately ASTM #12-14), residual carbide particles containing small amounts of V and Cb, and a small volume fraction of martensite in the bottoms formed from partial high-temperature austenitization (and rapid cooling in continuous annealing).The combination of solid solution strengthening from high Mn and Si content, the very fine grain size, the precipitation hardening effect of the carbide particles, and the presence of the second martensite phase all contribute to producing a steel with a very high work-hardening rate and a still relatively low yield strength (comparable to regular low-carbon 1008 / 1010 steel). The typical tensile properties of this steel, cold-rolled and heat-treated as described above, are as follows: Elastic limit - 40-50 KSI. Tensile strength - 80-100 KSI (typically 88-95 KSI) Elongation - 20-30% The fact that the tensile strength is almost twice as high as the yield strength indicates the high work-hardening characteristics of this material. Annealed bronze generally exhibits the same effect, with tensile strength being approximately 2 to 2.5 times that of the yield strength. This material, when severely pressed into the cartridge sidewall, will produce the high yield strength necessary for sufficient springback (successful extraction) and sufficient strength to prevent sidewall separation. In process route #2, the steel is continuously annealed at temperatures ranging from approximately 760°C (1400°F) to approximately 968.3°C (1775°F) to soften the final cold-rolled steel. The final annealing is the same as for process route #1, i.e., continuous annealing at approximately 968.3°C (1775°F), rapid cooling to approximately room temperature (1–2 minutes), and recoil at the furnace exit end. Process route #2 can produce a structure virtually identical to that produced in process route #1, although it offers some advantages, as will be further described. Steel produced in process route #1 typically exhibits a significant degree of ear stretching or planar anisotropy (due to crystalline preference). Ear stretching can result in a slightly non-uniform thickness around the circumference of the cartridge.By using the intermediate annealing step and final thickness reduction in process route #2, this tendency toward ear stretching can be reduced or minimized. Furthermore, this step can also tend to keep the yield strength toward the lower end of the variation due to a very slight coarsening of the ferrite grain size. Although both process routes #1 and #2 are effective in producing steel that performs well for rimfire cartridges, steel processed or treated using process route #2 may tend to form more consistently due to the minimized ear stretching and slightly less thickness variation around the cartridge circumference. In certain embodiments, the invention provides a treatment for a high-strength, low-alloy steel, including continuous high-temperature annealing to produce a dual-phase steel. In certain other embodiments, the invention provides a treatment for a grade 409 or 410 stainless steel, including low-temperature batch processing. The use of these materials is advantageous because a corrosion-resistant coating is not required after the steel treatment. Treated steel can be produced with minimal ear-stretch properties, similar to the processing of bronze. While not intended to be linked by any particular theory, it is believed that this allows for easier manufacturing or greater uniformity in the cartridge wall. Furthermore, it can introduce coarser grain size, resulting in a lower yield strength, for example, to facilitate rimfire, and a slightly higher rate of work hardening when the steel is formed into a cartridge. The steel can be pre-plated for corrosion resistance to provide extra lubrication during the drawing process and minimize tool wear. Both bronze and zinc plating with a special clear chromate (for extra protection against white rust with a zinc coating) can be used. Other plating types such as copper, cadmium, nickel, nickel-zinc, or any others that provide lubrication and are easy to remove for forming can also be used. However, some of these can be prohibitively expensive. In certain applications, steel can be plated for forming to provide extra lubrication and reduce tool wear. Alternatively, steel can be plated for corrosion resistance after ammunition cases are formed. Anyone experienced in the technique will appreciate that the annealing process specifications, including time periods and temperatures, can vary depending on the type of equipment used, among other factors. Alternative annealing parameters can be used to achieve the same result. EXAMPLES Testing the mechanical properties of bronze vs. steel The mechanical properties of the wall of an existing bronze ammunition cartridge were determined to identify the properties a steel composition must possess to match, or closely approximate, the elastic recovery of bronze. Rimfire cartridges are basically drawn and press-stretched, with press-stretching mainly occurring in the final forming stages. Because it was essentially impossible to sample the cartridge wall and measure the tensile properties, a method to approximate this was needed. Press-stretching most closely resembles cold rolling (for the simplest mechanical working process). Sections of the cartridge walls were measured for thickness, and then strips of annealed bronze were cold-rolled to the same thickness (the bronze cartridge walls after drawing and press-stretching ranged from approximately 0.030 cm to 0.020 cm (0.0.012” to 0.008”) from near the base of the cartridge to the open end, and therefore, the same thickness was used to roll the strips). Because the bronze used for rimfire cartridges is typically about 0.050 cm (0.020’j) thick, depending on the producer, the low-carbon steel and bronze-plated strips were originally rolled from material about 0.050 cm (0.020’j). All bronze and low-carbon steel strips at initial values ​​were in the normally used annealed condition (the bronze was obtained directly from a cartridge manufacturer, and the steel was standard 1008 / 1010 low-carbon batch annealed steel). Tensile tests were performed on strips of cold-rolled bronze and low-carbon steel to determine the yield strength, tensile strength, and elongation. The results, by comparing the bronze with the properties of the low-carbon steel, were used to develop special high-strength steels with the following compositions: from approximately 0.03 to approximately 0.18 wt% carbon, from approximately 0.15 to approximately 1.60 wt% silicon, from approximately 0.60 to approximately 2.50 wt% manganese, from more than 0 to approximately 0.025 wt% phosphorus, from more than 0 to approximately 0.025 wt% sulfur, and from approximately 0.20 to approximately 0.08 wt% aluminum and less than approximately 0.22 percent by weight of one or more of the metallic elements, based on the total weight of the composition. After processing the high-strength steel samples, strip samples were taken and cold-rolled to the same thickness, or approximately the same thickness as the bronze and low-carbon steel, for comparison purposes. The results of the cold-rolling experiments are shown later in Table 1. Only the yield strengths (in ksi) are shown because this is what was used to determine the springback. Table 1 Annealed Material @ 0.030 cm (0.012”) @ 2.254 cm (0.10”) @ 0.020 cm (0.008”) Bronze Cartridge 21.0 78.8 83.5 89.8 Reg. Low Carbon 48.0 88.1 91.5 95.7 High Strength, Continuous Annealed 48.0 134.6 140.3 152.1 High Strength Batch Annealed #1 44.2 115.3* 131.8 143.7** High Strength Batch Annealed 31.8 86.0*** 105.6 **★* resistance #2 The actual thickness was 0.034 cm (0.0134”) The actual thickness was 0.023 cm (0.0094”) The actual thickness was 0.032 cm (0.01275”) The rolling mill might not reach 0.020 cm (0.008”) Table 1 shows that the yield strength in the bronze sidewalls ranged from approximately 79 to approximately 90 KSI, while that of regular low-carbon steel was approximately the same. Due to the difference in elastic modulus between bronze and steel, this means that low-carbon steel could "shrink" back to half its original size as bronze. This is why extraction problems arise when ordinary low-carbon steel is used to produce cartridges.However, the continuously annealed high-strength steel, as shown in Table 1, was work-hardened to a range of approximately 135 to approximately 142 KSL. Even though this is slightly less than half the numbers for bronze, firing tests of cartridges made from this steel at the manufacturer showed no extraction problems in various types of handguns, including revolvers and semi-automatic pistols. One objective method used by the manufacturer to test extraction is to fire six cartridges in a revolver and then measure the actual force required to eject all six cartridges using a hand-operated extractor rod. This was performed with some cartridges made from the continuously annealed steel. The bronze was first fired to a baseline value, and in this three-shot pistol firing, the results averaged 0.82 kg (2.2 lb).The steel cartridges averaged under a force of approximately 0.005 kg (1.5 lb.) with no failures or sidewall separations. However, a few failures occurred with some other types of pistols. Thus, in revolver tests, the annealed steel cartridges consistently performed as well as the bronze. Failures can occur because the yield strength of bronze is significantly lower than that of steel. Therefore, the firing pin deformed the steel to a lesser degree than the bronze, resulting in less deformation for the primer. To minimize this effect, the priming mixture can be slightly adjusted for strain sensitivity, or the steel can be made slightly thinner to allow for more strain.These rimfire cartridges are made from a cup, pull, and head method, as well as the progressive die-head method, and are zinc-plated. The results shown in Table 1, for the continuously annealed high-strength steel and the firing tests with cartridges made of this steel, were obtained by heat-heating the steel according to Figure 4, process route #2. This steel had the following composition: from approximately 0.05 to approximately 0.10 carbon, from approximately 0.20 to approximately 0.50 silicon, less than approximately 0.07 chromium, from approximately 0.70 to approximately 1.45 manganese, from approximately 0.05 to approximately 0.14 vanadium, less than approximately 0.05 nickel, less than approximately 0.02 phosphorus, from approximately 0.04 to approximately 0.12 niobium, less than approximately 0.03 molybdenum, less than approximately 0.016 sulfur, and from approximately 0.02 to approximately 0.08 aluminum, based on the total weight of the composition. Table 1 shows that the batch-annealed high-strength steel #1, hardened for strain relief to levels between approximately 115 KSI and approximately 144 KSI, was similar to the continuously annealed high-strength steel. The yield strength was also close to that of the continuously annealed steel at approximately 44 KSI. Rimfire cartridges were successfully manufactured from this steel at the rimfire manufacturing plant and tested for firing. The same revolver test as previously discussed was also used with these cartridges. There were significant sidewall separations with some of these cartridges. Upon examination, it appeared that cracks were emanating from die scratches around the entire tips of the cartridges. It should be noted that this steel was supplied in a trial quantity and was not plated.When the gaps did not inhibit the extractor's "pulse," extraction force levels of approximately 0.82 kg (2.2 lb) were measured. Thus, the resistance level in the sidewalls of cartridges made from this steel was sufficient to allow for good extraction. It is anticipated that the plating will result in much better lubrication of the extraction dies, and this could eliminate the tendency for die scoring. The lubrication normally used for bronze is likely to be the same. IVIA / a / ZUZZ / UI 4114 was not optimal for metallic white steel. There were also some failures in this test. The analysis in the previous paragraph also applies to this material. These cartridges are made using the cup, extraction, and head method. The results shown in Table 1 for high-strength batch annealed steel #1, and the firing tests with cartridges made from this steel, were obtained by heat-treating the steel according to Figure 3C. This steel had the following composition: from approximately 0.16 to approximately 0.18 percent carbon, from approximately 1.25 to approximately 1.55 percent silicon, from approximately 1.9 to approximately 2.1 percent manganese, approximately 0.02 percent phosphorus, approximately 0.02 percent sulfur, from approximately 0.025 to approximately 0.055 percent aluminum, less than approximately 0.06 percent copper, less than approximately 0.04 percent nickel, less than approximately 0.06 percent chromium, and less than approximately 0.02 percent molybdenum based on the total weight of the composition. Table 1 shows that high-strength batch annealed #2 steel strain hardened to levels of approximately 86 KSI to 106 KSI, lower than for high-strength batch annealed #1 steel and high-strength continuous annealed steel. However, it is noted that the highest level here was for the material at 0.025 cm (0.010”) and not 0.020 cm (0.008”). The rolling mill available for rolling this material might not achieve 0.020 cm (0.008”). Extrapolating the data would place the yield at 0.020 cm (0.008”) at approximately 115–120 KSI. Also, part of the reason for the somewhat lower values ​​is the lower silicon level for the high-strength #2 batch annealed steel. This chemistry was used to obtain a lower value in an attempt to determine the lowest yield strength capable of producing acceptably extractable cartridges, and it provides a lower tendency for failure (due to the lower yield strength as annealed).Rimfire cartridges were successfully manufactured from this steel, although a small amount of ear stretching was present. Firing tests with this high-strength batch annealed #2 steel were conducted concurrently with the firing tests for the high-strength #1 material described above. The initial firing test value for the bronze was 0.82 kg (2.2 lbs), the force for 3 pistol loads. The results for the high-strength batch annealed #2 steel averaged 0.91 kg (2.46 lbs) of force, and there were no failures; thus, this material was a perfect match for the bronze.Thus, it has been determined that the necessary yield strength in the side walls of the rimfire cartridge, allowing for good extraction, can be significantly less than twice the level of bronze, although these levels can only be achieved with specially selected and processed steels. These cartridges were produced using the cup-and-pull method. The results shown in Table 1 for batch annealed steel, with a high strength #2, and the firing tests with cartridges made from this steel, were obtained by processing the steel according to Figure 3C. This steel had the following composition: from approximately 0.126 to approximately 0.154 percent by weight of carbon, from approximately 0.395 to approximately 0.605 percent by weight of silicon, from approximately 1.75 to approximately 1.95 percent by weight of manganese, approximately 0.02 percent by weight of phosphorus, approximately 0.005 percent sulfur, from approximately 0.02 to approximately 0.06 percent by weight of aluminum, less than approximately 0.06 percent by weight of copper, less than approximately 0.04 percent by weight of nickel, less than approximately 0.06 percent by weight of chromium, and less than approximately 0.02 percent by weight of molybdenum, based on the total weight of the composition. While the specific modalities of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the general teachings of the description. Therefore, the particular modalities described should be understood to be illustrative only and not limiting to the scope of the invention, which will provide the full scope of the appended claims and any and all equivalents thereof.

Claims

1. A steel composition for rimfire ammunition cartridges, characterized in that it comprises: from approximately 0.16 to approximately 0.18 percent by weight of carbon; from approximately 1.25 to approximately 1.55 percent by weight of silicon; from approximately 1.9 to approximately 2.1 percent by weight of manganese; approximately 0.02 percent by weight of phosphorus; approximately 0.02 percent of sulfur; from approximately 0.025 to approximately 0.055 percent by weight of aluminum; copper present in an amount less than approximately 0.06 percent by weight; nickel present in an amount less than approximately 0.04 percent by weight; chromium present in an amount less than approximately 0.06 percent by weight; and molybdenum present in an amount less than approximately 0.02 percent by weight, based on the total weight of the composition.

2. The composition according to claim 1, characterized in that it comprises: from approximately 0.126 to approximately 0.154 percent by weight of carbon; from approximately 0.395 to approximately 0.605 percent by weight of silicon; from approximately 1.75 to approximately 1.95 percent by weight of manganese; approximately 0.02 percent by weight of phosphorus; approximately 0.005 percent by weight of sulfur; from approximately 0.02 to approximately 0.06 percent by weight of aluminum; copper present in an amount of less than approximately 0.06 percent by weight; nickel present in an amount of less than approximately 0.04 percent by weight; chromium present in an amount of less than approximately 0.06 percent by weight; and molybdenum present in an amount of less than approximately 0.02 percent by weight, based on the total weight of the composition.

3. A method for processing a steel for use in a rimfire cartridge, the method being characterized in that it comprises: obtaining a steel composition having an original thickness; cold rolling the steel composition to reduce the original thickness by at least 70% to produce a cold-rolled steel composition having an intermediate thickness; first annealing and then quenching the steel composition of intermediate thickness to produce an annealed intermediate steel composition; cold rolling the annealed intermediate steel composition to a thickness that is reduced by approximately 20% to approximately 35% of the intermediate thickness of the intermediate steel composition to produce a steel composition having a final thickness;performing a second annealing and subsequent cooling of the steel composition to a final thickness to produce a final annealed steel composition of a final thickness; continuous cladding of the final annealed steel composition of a final thickness.

4. The method according to claim 3, characterized in that the first and second annealings are conducted as a batch process.

5. The method according to claim 3, characterized in that the first and second annealings are conducted as a continuous process.

6. The method according to claim 3, characterized in that the continuous plating is carried out before the second cold rolling step.

7. The method according to claim 6, characterized in that the continuous plating performed before the second cold rolling step is in addition to the continuous plating performed after the second annealing.

8. The method according to claim 6, characterized in that the continuous plating performed before the second cold rolling step is in place of the continuous plating performed after the second annealing.

9. The method according to claim 3, characterized in that the continuous plating may include depositing an element selected from the group consisting of zinc, bronze, and combinations and alloys thereof.

10. A method for processing a steel composition to form a rimfire cartridge, characterized in that it comprises: obtaining a steel composition having an original thickness; cold rolling the steel composition to produce a steel composition having a final thickness; annealing and subsequently cooling the steel composition having a final thickness to produce a final annealed steel composition having a final thickness; and continuous plating of the final annealed steel material having a final thickness.