Patent Application
20090038718
The present invention relates to a process for the manufacture of a containment device and a containment device manufactured by said process and to a method of producing an isolation barrier material for producing said containment devices.
Containment devices are used to separate a first environment from a second environment where it is important that both environments do not come into contact with one another and where it is important that the environmental conditions in the first environment can be contained for a short period of time e.g. in case of a rimfire cartridge, or for a sustained period of time e.g. in case of an engine gasket. In a containment device such as an engine gasket, which is for instance positioned between a cylinder head and a cylinder block, which jointly define the combustion chamber of an automotive engine, the internal environment of the engine is separated from the outside environment. An engine gasket is a sealing member having an opening, which generally has a circular shape with essentially the same diameter as the cylinder of the engine, and an annular bead, which is a ridge, formed by beading so as to surround the opening. The bead functions as a macro-seal since it is compressed between the cylinder head and cylinder block and seals the interstice there between to prevent leakage of combustion gas from the combustion chamber, cooling water from cooling means for cooling the engine, and lubricating oil from lubrication means for lubricating moving parts of the engine. Additional micro sealing is provided by an elastomer, such as a fluoro-elastomer. The gasket functions as a containment device because its main purpose is to contain the different media (such as gas, oil, water) in their proper environments. It is important to note that a containment device is meant to act as a separator device not only under static conditions, but also under dynamic conditions.
A material for fabricating such an engine gasket is therefore required to have high strength (high hardness and high yield stress) sufficient to retain a bead against compression, along with good workability, adequate corrosion resistance and thermal stability, but also requires adequate formability during forming of the gasket. It should also be noted that the fatigue properties of containing device such as an engine gasket are of major importance because the gasket is loaded and unloaded at each explosion in any of the combustion chambers enclosed by the cylinder, the piston and the cylinder head. A further requirement is a low anisotropy of the properties of the material from which the gasket is formed. It should be noted that when the term anisotropy is used, this is to be understood as planar anisotropy. Thermal stability of the properties is important as well because in operation, temperatures of the gasket may be high, for instance about 110° C.
In a containment device such as a cartridge for ammunition for firearms it is important to contain the internal exploding atmosphere in the cartridge from the outside atmosphere as long as possible in order to obtain a maximum transfer of energy from the explosion to the bullet leaving the barrel of the firearm. During the explosion the cartridge has to ensure sealing of the backside of the barrel of the firearm to prevent energy loss by escaping gas at the backside. This sealing is ensured by an elastic expansion of the cartridge during the explosion. After the explosion the cartridge relapses allowing easy removal of the cartridge from the barrel. A good formability during forming of the cartridge is required, in combination with a high yield strength of the final cartridge. A high yield strength results in the required elastic expansion of the cartridge during the explosion without plastic deformation of the cartridge, which could cause it to stick in the barrel. A further requirement is a low planar anisotropy of the properties of the material from which the cartridge is formed to prevent or reduce earing during forming of the cartridge.
In a containment device such as a battery case it is important to contain the often harmful and corrosive content of the battery from the outside environment to prevent corrosion, pollution or health risks. To improve the capacity of a battery, it is important to increase the volume of the battery case without changing the external dimensions of the battery. Down gauging the wall thickness of the case would result in such a volume increase. However, in a battery, the internal pressure may easily exceed a value of 30 bar. This high pressure must not result in plastic deformation or failure of the battery. Hence a high strength and a high yield point is required of the battery case, but a low yield stress and high formability is required during the forming of the case by draw-and-wall-ironing (DWI) or draw-and-redraw (DRD) techniques. A further requirement is a low anisotropy of the properties of the material from which the cartridge is formed to prevent or reduce earing during forming of the battery case.
In order to meet the above-described requirements for engine gaskets, a known solution is to use a metastable austenitic stainless steel, such as SUS 301 stainless steel, which is a Cr- and Ni-added stainless steel. Deformation of such a steel by cold working, such as cold rolling and beading, causes the metastable austenite in the deformed area to transform to martensite, which has a greater hardness. Thus, the steel can exhibit a high work hardenability with good initial workability.
However, such a stainless steel has the disadvantage that its properties, particularly hardness, may fluctuate greatly, since the increased hardness of the steel obtained by working may vary significantly depending on the working ratio of the steel and the temperature at which the steel is subjected to working. Therefore, the quality, particularly the sealing quality of gaskets made from the steel, may fluctuate significantly. Another disadvantage is that the metastable austenitic steel is susceptible to stress corrosion cracking. Furthermore, the steel contains a large amount of nickel, which is expensive, thereby adding to the production costs of the gaskets.
In order to cope with these problems, a Cr-based martensitic stainless steel having a tempered martensitic structure has been proposed for the fabrication of engine gaskets in JP 7-278758. In general, martensitic stainless steels have an improved resistance to stress corrosion cracking over the metastable austenitic stainless steel described above. Moreover, it is relatively easy to achieve a high hardness with martensitic stainless steel by means of quenching from a high temperature, which causes transformation to form hard martensitic phases. Furthermore, martensitic steel is less expensive since it contains a lower nickel content.
However, since martensitic stainless steels in the as-quenched condition have a decreased elongation and are difficult to work, it is essential that the quenched martensitic steel be subjected to heat treatment to a tempering heat treatment after quenching. Such heat treatments add to the production costs of the steel and may cause embrittlement of the steel due to formation of carbides or a loss of corrosion resistance due to the formation of Cr-deficient phases resulting from the formation of carbides.
The application of stainless steel for gaskets has a number of disadvantages. Firstly, the costs of stainless steel are high due to the high level of expensive alloying elements such as for instance Chromium and Nickel. Furthermore, the final properties of a stainless steel are very sensitive to the processing conditions and these processing conditions are quite demanding. In addition, stainless steels contain many alloying elements in significant quantities. As a result of non-uniform distribution of the alloying elements and the effect of fluctuations in processing conditions thereupon, the mechanical properties of these steels also fluctuate significantly. Therefore, the reproducibility and the anisotropy of the mechanical properties of stainless steels is a constant concern.
It is known that the yield strength of a steel can be increased by subjecting it to a second cold rolling at a draft of 30% or more. The disadvantage of this second cold rolling step at a draft of 30% or more is the large anisotropy, which is the result of the second cold rolling.
It is an object of this invention to provide a process for the manufacture of a containment device with a high post-manufacture yield strength
It is another object of this invention to provide a process for the manufacture of a containment device with a reduced anisotropy of properties.
It is another object of this invention to provide a process for the manufacture of a containment device made from an economically attractive material.
It is still another object of this invention to provide a process for the manufacture of a containment device with a reduced sensitivity of the properties to the processing conditions.
According to the invention, one or more of these objectives are achieved with a process for the manufacture of containment device manufactured comprising the steps of:
With a process for the manufacture of a containment device according to the invention, there is no need for a very costly alloy basis such as in the case of a stainless steel. The steel basis according to the invention, subjected to the process as described hereinabove, will result in a steel strip with a microstructure which comprises at least 10% in volume of at least one phase selected from a group of phases consisting of martensite and bainite. These phases contribute greatly to the strength of the material. Due to the annealing above Ac1, the anisotropy of the final product is greatly reduced because the phase transformation from ferrite to austenite upon heating and the subsequent re-transformation from austenite to the desired phases during rapid cooling randomises the texture of the material to a great extent, thereby reducing the anisotropy of the material.
Surprisingly, it was found that the containment device produced according to the invention has a high bake hardening potential. Upon heating the containment device, which has undergone deformation to form it, to a temperature of for example between 100 and 200° C. a very significant increase in yield strength could be observed. This also results in excellent fatigue properties.
Consequently, a bake-hardening treatment after forming the part further increases the yield strength of the material in the finished part. An advantage of the process according to the invention is that no second cold rolling treatment is required to achieve the desired final properties. An additional cold rolling step would significantly increase the anisotropy of the properties, which is undesirable for many containment devices. It should be noted that a second cold rolling treatment is to be understood as a rolling treatment involving a reduction of more than 10% since these levels of deformation will deleteriously affect the anisotropy of the product. Any cold rolling treatment involving a reduction of at most 10% is considered to be a temper-rolling treatment.
The containment device produced according to the invention therefore favourably combines high strength, excellent fatigue properties and a reduced anisotropy.
The cooling of the hot-rolled steel is optionally performed using accelerated cooling equipment such as a laminar cooling unit, or an ultra fast cooling unit, both units mainly using water as a coolant, but it could also be performed using a mist cooling unit or a gas-cooling unit. Typical cooling rates during accelerated cooling would be between 10 and 200° C./s, although using a cooling of the ultra fast cooling type, the cooling rate could be significantly higher, up to 1500° C./s per unit thickness (in mm) (i.e. 500° C./s for a 3 mm strip).
The mechanical properties of the containment device can be further tuned in embodiments of the invention wherein the chemical composition of the steel also comprises, on a weight basis,
Chromium and molybdenum are ferrite stabilising elements, raise the transformation temperature from austenite to ferrite (A3) and retard decomposition of austenite by slowing down the diffusivity of carbon in austenite. Vanadium, titanium and niobium have the same effect. All mentioned elements are also strong carbide formers, resulting in a precipitation of carbides under the proper thermo-mechanical conditions (i.e. temperature, strain and strain rate). The addition of these elements consequently allows tuning the microstructure of the steel as well as the mechanical properties, resulting in a containment device with the desired properties to perform its function.
In an embodiment of the invention, the silicon content of the steel is at most 1.0%, preferably at most 0.5%. By reducing the silicon content, the condition of the surface of the material improves.
In an embodiment of the invention the cold rolling reduction is between 50 and 95%, preferably between 70 and 90%, more preferably between 75 and 88%. The anisotropy of the final cold-rolled and annealed product at least partly depends on the amount of cold rolling reduction. In combination with the annealing treatment it was found that the cold-rolling reduction should preferably be at least 50% but more preferably be at least 75%. Although the cold rolling reduction of step c. is preferably brought about in one process step, for instance in a multi-stand rolling mill or in a reversible cold-rolling mill, step c may also consist of two separate cold rolling steps with an intermediate recrystallising annealing between the two separate cold rolling steps. This is particularly relevant for less powerful cold rolling mills. However, the total cold rolling deformation is the same as for the single cold rolling step.
In an embodiment of the invention the steel in the containment device comprises at least 20% in volume of a martensite phase, the remainder preferably comprising at least 60% in volume of ferrite. The increase in martensite content ensures a further increase in strength. In this embodiment, the resulting structure is commonly referred to as a dual-phase structure, although other phases like bainite and/or retained austenite are known to be possible in these steels, albeit in quantities not affecting the beneficial properties associated with the dual-phase steel. In this embodiment, the ferrite content needs to be at least 60% to ensure sufficient hardness of the martensite phase. During annealing and upon transformation during cooling carbon is rejected from the ferrite and concentrates in the remaining austenite. If the carbon enrichment is sufficient and the cooling rate is high enough, the austenite may transform to martensite upon further cooling. The hardness of the martensite depends at least partly on its carbon content. The formability of the dual-phase structure is excellent and the presence of the martensite embedded in the ferritic matrix ensures a low initial yield stress, whereas the ultimate strength of the material is high. After deformation, i.e. after forming the containment device, the yield strength has increased significantly thereby increasing the potential of the material to accommodate elastic stresses, because plastic deformation does not occur until the increased yield stress is exceeded.
In an embodiment of the invention the steel in the containment device comprises at least 80%, preferably at least 90% in volume of a martensite phase. This very high level of martensite ensures a very high strength, and a very high yield stress. Although the hardness of the martensite phase itself decreases with increasing martensite fraction due to the lower carbon content in the martensite, the large amount of martensite still ensures a strong increase in strength. To achieve this high martensite content, the continuous annealing of step d has to be performed by reheating to a temperature near Ac3 or even above Ac3. After forming the containment device the very high yield strength ensures a very high potential of the material to take up elastic stresses, because plastic deformation does not occur until the yield stress is exceeded. The higher the martensite content, the higher the strength of the material, usually at the expense of the formability. In cases where limited formability is required, but a very high elastic potential, the required martensite content could be 90% or even higher. A fully martensitic steel would ensure a very high strength. For applications where only limited formability is required and a very high elastic potential, a containment device formed from a steel with 90% in volume of a martensitic phase, or even a fully martensitic structure, would be suitable.
Surprisingly, it was found that the containment device produced according to this embodiment invention has a very high bake hardening potential. Upon heating the containment device, which has undergone deformation to form it, to a temperature of for example between 100 and 200° C. a very significant increase in yield strength could be observed. This results in a significant advantage over a containment produced from a conventional material such as stainless steel, which have to be subjected to an temperature of about 400° C. to achieve the desired level of precipitation. This implies that the material has to be subjected to an additional process step. In addition, the curing of the elastomer, which is used for additional microsealing, has to take place in a separate process step at temperatures of between 100 and 200° C. When subjecting a formed part of the material according to the invention to said curing step, bake hardening of the material occurs, thereby resulting in an increase of the yield strength after forming the part. It should also be noted that a temperature at the location of a gasket of between 100 and 200° C. in an engine readily occurs. Therefore the use of said material in an engine gasket results in a further increase of the yield strength without the need for an additional process step. This increase of the yield strength is an isotropic increase because the bake-hardening effect is isotropic.
Consequently, a bake-hardening treatment after forming the part further increases the yield strength of the material in the finished part.
In an embodiment of the invention the steel is coated with a metallic coating. This may be done before or after the continuous annealing, partly depending on the type of coating. Although the coating may be provided using a process such as PVD, in a preferred embodiment the coating is applied by electroplating, preferably prior to continuous annealing. In case the steel is also subjected to a second cold-rolling step, the electroplating step may take place prior to or after the second cold rolling step. The type of metal or metals chosen for the metallic coating depends on the specific requirements of the containment device and the environmental conditions in which it is to function. In an embodiment the metallic coating is selected from a group of metallic coatings consisting of Cu, Ni, Co, Al, Zn, Ti, Cr or alloys thereof. In a preferred embodiment of the invention, the metallic coating is a barrier coating such as a nickel-based coating, such as a nickel coating preferably with a minimum nickel-content of at least about 85%. A nickel coating is a very versatile coating, which provides the steel strip with protection against the corrosive properties of the environment, even at high temperatures. In another embodiment the metallic coating is sacrificial to steel such as nickel-zinc or zinc.
In addition to, or instead of, a metallic coating it is also possible to provide the containment device with an organic coating for reasons of corrosion resistance or lubrication purposes.
In order to increase the hardness of the surface of the containment device, a carburizing step or nitriding step may be part of the process for manufacturing a containment device.
In an embodiment of the invention the total nitrogen content of the steel is greater than 5 ppm and/or not greater than 150 ppm, preferably wherein the total nitrogen content is between 15 and 125 ppm, more preferably wherein the total nitrogen content between 25 and 100 ppm. The amount of nitrogen enables to control the bake hardening behaviour.
In any of the embodiments, the steel may be temper rolled to provide the desired surface quality, roughness, shape or mechanical properties wherein the temper rolling reduction is 10% or less, preferably 8% or less, more preferably 5% or less, even more preferably less than 3%. The temper rolling reduction is preferably at least 1.5%, more preferably at least 2%. At temper rolling reductions of above 10%, the anisotropy caused by the cold-rolling step increases rapidly to unacceptable levels. The temper rolling treatment, which may be replaced by a tension-levelling treatment, produces the amount of cold deformation in the material to benefit optimally from the bake-hardening potential of the material. The lower boundary value for the temper rolling reduction is applied to achieve a homogeneous bake-hardening effect, because if the temper rolling reduction is too low or even zero, then the difference in bake-hardening effect between the deformed parts of a formed part such as the bead in a gasket and the undeformed part will become too large, resulting in a higher susceptibility to fatigue of the formed part, and in a lower base yield strength of the formed part.
According to a second aspect of the invention, a containment device is provided which is manufactured according to the process as described hereinabove. Depending on the volume of at least one phase selected from a group of phases consisting of martensite and bainite, this containment device provides a high post-manufacture yield strength and/or a reduced anisotropy of properties which is made from an economically attractive material and which provides a reduced sensitivity of the properties to the processing conditions.
In an embodiment of the invention the containment device is a gasket for use in an internal combustion engine.
The present inventors found that when producing a containment device such as a gasket as described hereinabove, the containment device having a high post-forming yield stress is obtained. When the gasket is produced according to the embodiment wherein the steel of the gasket comprises at least 60% of a ferrite phase, the pre-forming yield stress is low, and the post-forming yield stress has increased with respect to the pre-forming yield stress. When the gasket is produced according to the embodiment wherein the steel of the gasket comprises at least 80% of a martensite phase the pre-forming yield stress is already high and the post-forming yield stress has increased with respect to the pre-forming yield stress. This high post-forming yield stress results in good fatigue properties of the gasket with a low anisotropy value. These fatigue properties are important because of the cyclic loading of the gasket due to the repeated explosions in the combustion chamber, or chambers of the engine. In case the steel is coated with a suitable metallic coating, the gasket possesses a corrosion resistance comparable to stainless steel gaskets. The post-forming yield strength and/or overall strength may be further increased by the bake-hardening potential of the material. The inventors surprisingly found that a containment device according to the invention, particularly if the containment device comprises a large fraction of martensite, such as at least 80%, has a very large bake-hardening potential. This bake hardening may take place ex-situ (i.e. before application of the gasket in the engine) or in-situ (i.e. after application of the gasket in the engine). In the latter case the heat developed in the engine enables the bake-hardening process to occur. Since the amount of deformation during production of the gasket is limited and localised, preferably the material to be formed into a gasket has been subjected to the temper rolling or tension levelling treatment to promote the bake-hardening effect as described hereinabove. This is also important to limit the difference in bake hardening effect between the deformed and undeformed parts of the gasket after forming the gasket.
In an embodiment of the invention the containment device is a cartridge for rimfire ammunition. Such a cartridge can be formed from a sheet metal by drawing a cup and further forming into a cartridge. In this type of process it is important that the material has a low earing property, i.e. a low in-sheet or planar anisotropy of the properties of the material. After forming the cartridge, the material of the wall of the cartridge is heavily deformed. This material consequently has a very high yield stress. When using the cartridge as a rimfire cartridge in a firearm, the cartridge undergoes a very large load transient upon explosion of the explosive in the cartridge. During the explosion the cartridge has to ensure sealing of the backside of the barrel of the firearm to prevent energy loss by escaping gas at the backside. This sealing is ensured by an elastic expansion of the cartridge during the explosion. After the explosion the cartridge relapses allowing easy removal of the cartridge from the barrel. The high post-forming yield strength results in the required elastic expansion of the cartridge during the explosion without plastic deformation of the cartridge, which could cause it to stick in the barrel. Since the amount of deformation during production of the cartridge is significant, the importance of the temper rolling or tension levelling treatment to promote the bake-hardening effect as described hereinabove is reduced but it may be relevant for the flatness or roughness of the material.
In an embodiment of the invention the containment device is a case for a battery. Such a battery case can be formed from a sheet metal by drawing a cup and further forming into a case, a process not unlike the process for forming a cartridge for ammunition. Also for battery cases it is important that the material has a low earing property, i.e. a low in-sheet anisotropy of the properties of the material. The higher the earing, the more material needs to be trimmed from the case (or cartridge) after the final forming step. After forming the case, the material in the wall of the case is heavily, deformed. This material consequently has a very high yield stress. The material in the bottom of the cup has undergone much less deformation, particularly in a DWI process, resulting in a larger remaining thickness of the bottom. When using the case to form a battery, the battery has to withstand a very high internal pressure, without bulging. If the battery bulges, the appearance of the battery is compromised and the larger diameter as a result of the bulging could also cause the battery to get stuck in the battery compartment of an apparatus such as a torch. If the battery bulges at the bottom, the appearance of the battery is compromised and it may cause the battery to get stuck in a battery compartment. Because of the high yield stress of the walls, the thickness of the battery wall can be reduced, thus enabling a higher capacity of the battery. This is the result of the larger internal volume of the case. The higher strength of the walls compensates for the lower thickness of the case. Since the amount of deformation during production of the case is significant, the importance of the temper rolling or tension levelling treatment to promote the bake-hardening effect as described hereinabove is reduced but it may be relevant for the flatness or roughness of the material.
In general, the containment device according to the invention is produced from an isolation barrier material for high temperature and/or high pressure sealing applications. The invention is therefore also embodied in a process for manufacturing said isolation barrier material according to the process described hereinabove.
The thickness of the steel strip after the last and final rolling step as described hereinabove preferably is below 1.5 mm, preferably below 0.75 mm. The thickness is preferably at least 0.10 mm. The preferable thickness range depends on the type of application. For the application of the containment device as a gasket the preferable thickness range is between 0.15 and 0.60 mm. For the application of the containment device as a rimfire cartridge the preferable thickness range is between 0.35 and 0.50 mm and for the application of the containment device as a battery case the preferable thickness range is between 0.15 and 0.25 mm,
Specific embodiments of the present invention will now be explained by the following non-limitative examples.
The following steels were continuously cast and hot rolled to 4.5 mm (Ex. 1) or 3.0 mm (Ex. 2) according to a conventional hot rolling schedule in an 88 inch hot strip mill followed by cold rolling to 0.96 mm in an industrial tandem cold-rolling mill.
These steels were subjected to three types of annealing treatment in step d.:
Additional experiments were performed starting from a 2.0 mm hot rolled steel strip with a composition according to Ex. 2. The values for the r-value and Δr-value were determined in the usual way from measurements of samples taken at angles of 0°, 45° and 90° to the rolling direction.
Cooling factor is a measure for the cooling rate after annealing. The higher the cooling factor, the higher the cooling rate. Typical cooling rates obtained in these experiments were between about 100 and 200° C./s. The hot rolled strip had a thickness of 2.0 mm. It is apparent that the higher the cold rolling reduction, the higher the strength. Also, the higher the cooling factor, the higher the cooling rate, and hence the higher the strength.
From Table 5 it is apparent that the strength increases with increasing cooling factor.
Containment devices produced from the material in Table 4 and 5 provided excellent containment performance in any of the aforementioned applications, such as engine gaskets. A very high post-forming yield strength was combined with a low anisotropy and excellent fatigue properties.
It is clear from the data of Table 6 that the bake-hardening effect in the Martensitic matrix Multiphase materials is particularly strong. The total effect of the 2% work hardening and the bake-hardening treatment amount to values over 300 MPa for MM B (344 MPa) and MM C (400 MPa).
It is clear from the data of Table 7 that the anisotropy in yield strength observed for the as annealed martensitic matrix multiphase steel is much lower than the values obtained for the 50% cold rolled FMDP E (i.e. the cold rolled condition which exhibits comparable strength to that of the annealed MM C/0yield strength variation observed for the as annealed variant is of the order of 20-45 MPa rather than the 100-200 MPa reported for the 50% cold rolled DP. It is also clear that the anisotropy is maximal at a cold deformation value of 20% and stabilises at higher deformations. The increase of yield strength in the first 10% of cold deformation is notable, whereas the further increase at higher values is less strong.
Combination of the results of Table 6 and 7 shows that the annealed martensitic matrix multiphase steel type already has a high isotropic base strength and adequate formability, whereas after it has been formed into a containment device such as a gasket it will produce a bake-hardening effect resulting in a further isotropic increase of the yield strength and hence excellent and isotropic fatigue properties. Subjecting the MM C/0 to a pre-deformation of 3% instead of 2% results in a WH value of 289 MPa and a BH3 value of 174 MPa.
A more detailed study of the influence of small deformations such as those occurring in temper rolling revealed that the amount of temper rolling should preferably be 10% or less, preferably 8% or less, more preferably 5% or less, even more preferably less than 3%. The amount of temper rolling is preferably at least 1.5% more preferably at least 2.0%. As shown in Table 7 temper-rolling values of above 10% result in a strong increase of the anisotropy caused by the cold-rolling step increases rapidly to unacceptable levels, whereas the increase in yield stress saturates very rapidly. So a combination of an increase in yield strength with low anisotropy and a good bake hardening potential is obtained by a temper rolling treatment within the range as given in this paragraph.
It is of course to be understood that the present invention is not limited to the described embodiments and examples described above, but encompasses any and all embodiments within the scope of the description and the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04077943.1 | Oct 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/11609 | 10/26/2005 | WO | 00 | 10/31/2008 |
