LONG LIFE MOLD TOOL STEEL WITH IMPROVED PHYSICAL PROPERTIES AT HIGH TEMPERATURE AND MOLD USING THE SAME

Abstract

The present invention relates to an alloy steel for a mold tool and a mold using the same.

The steel for the mold tool comprises iron (Fe) as a main component, an amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), and amount of about 0.80 to 1.20 wt % of vanadium (V) based on the total weight of the tool steel. Accordingly, the mold tool have improved physical properties at high temperature and extended life span.

Claims

1. An alloy steel for a mold tool, comprising: amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), amount of about 0.80 to 1.20 wt % of vanadium (V), and iron (Fe) constituting the remaining balance of the alloy steel, all the wt % based on the total weight of the alloy steel.

2. The alloy steel of claim 1, further comprising niobium (Nb).

3. The alloy steel of claim 2, wherein a content of the niobium (Nb) is about 0.05 to 0.10 wt % based on the total weight of the alloy steel.

4. The alloy steel of claim 1, further comprising tungsten (W).

5. The alloy steel of claim 4, wherein a content of the tungsten (W) is amount of about 0.10 to 1.00 wt % based on the total weight of the alloy steel.

6. The alloy steel of claim 1, further comprising niobium (Nb) and tungsten (W).

7. The alloy steel of claim 6, wherein a content of the niobium (Nb) is amount of about 0.05 to 0.10 wt % and a content of the tungsten (W) is amount of about 0.10 to 1.00 wt % based on the total weight of the alloy steel.

8. The alloy steel of claim 1, consisting essentially of: amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), amount of about 0.80 to 1.20 wt % of vanadium (V), and iron (Fe) constituting the remaining balance of the alloy steel, all the wt % based on the total weight of the alloy steel.

9. The alloy steel of claim 1, consisting essentially of: amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), amount of about 0.80 to 1.20 wt % of vanadium (V), amount of about 0.05 to 0.10 wt % of niobium (Nb), and iron (Fe) constituting the remaining balance of the alloy steel, all the wt % based on the total weight of the alloy steel.

10. The alloy steel of claim 1, consisting essentially of: amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), amount of about 0.80 to 1.20 wt % of vanadium (V), amount of about 0.10 to 1.00 wt % of tungsten (W), and iron (Fe) constituting the remaining balance of the alloy steel, all the wt % based on the total weight of the alloy steel.

11. The alloy steel of claim 1, consisting essentially of: amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), amount of about 0.80 to 1.20 wt % of vanadium (V), amount of about 0.05 to 0.10 wt % of niobium (Nb), amount of about 0.10 to 1.00 wt % of tungsten (W), and iron (Fe) constituting the remaining balance of the alloy steel, all the wt % based on the total weight of the alloy steel.

12. A mold comprising a steel of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 illustrates change in cavity of a mold according to the related art.

[0021] FIG. 2 shows breakage of a forging mold according to the related art.

[0022] FIG. 3 shows molybdenum carbide of an exemplary steel for an exemplary mold tool according to an exemplary embodiment of the present invention.

[0023] FIG. 4 is a graph illustrating an yield strength according to an exemplary content of molybdenum of an exemplary alloy steel for an exemplary mold tool according to an exemplary embodiment of the present invention.

[0024] FIG. 5 is a graph illustrating tensile strength according to an exemplary content of molybdenum of an exemplary alloy steel for an exemplary mold tool according to an exemplary embodiment of the present invention.

[0025] FIG. 6 is a graph illustrating hardness according to an exemplary austenizing temperature in an alloy steel composition according to Comparative Example 1 which is the related art and when tungsten is added to a mold tool steel according to Comparative Example 1 which is the related art.

[0026] FIG. 7 shows tungsten carbide of an exemplary alloy steel for an exemplary mold tool according to an exemplary embodiment of the present invention.

[0027] FIG. 8 is a graph illustrating toughness according to an exemplary content of tungsten of an exemplary alloy steel for an exemplary mold tool according to an exemplary embodiment of the present invention.

[0028] FIG. 9 illustrates progression of cracks according to the size of large crystal grains in the related art.

[0029] FIG. 10 illustrates progression of cracks according to the size of small crystal grains in the related art.

[0030] FIG. 11 is a graph illustrating stretching ratio according to an exemplary content of niobium of an exemplary alloy steel for an exemplary mold tool according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0032] Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about.”

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

[0033] Hereinafter, the exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, the terms or words used in the present specification and the claims should not be construed as being limited as typical or dictionary meanings, and should be construed as meanings and concepts conforming to the technical spirit of the present invention on the basis of the principle that an inventor can appropriately define concepts of the terms in order to describe his or her own invention in the best way. Accordingly, since the exemplary embodiments described in the present specification and the configurations illustrated in the drawings are given in an exemplary embodiment of the present invention and do not represent all of the technical spirit of the present invention, it is to be understood that various equivalents and modified examples, which may replace these exemplary embodiments and configurations, are possible at the time of filing the present application.

[0034] An aspect of the present invention relates to an alloy steel with improved physical properties at high temperature and extended life span. In general, when parts of the vehicle are manufactured, the durability of the parts may be improved and the strength may be increased, so that load imposed on a mold used to manufacture the parts may be increased. Furthermore, the load imposed on the mold may be increased due to an increase in the number of cavities of the mold as shown in FIG. 1 in order to improve the productivity of parts. For example, when the parts are usually manufactured using a forging press, a first process (buster) includes a process for distributing the volume, a second process (blocker) includes a process for rough-shaping, a third process (finisher) includes a process for dimension molding, a fourth process (piercing) includes a process for separating the inner flash, and a fifth process (trimming) includes a process for separating the outer flash. Among the processes, the high load processes may be the first to third processes. Due to the first to third processes as described above, the mold may be broken due to the high load as illustrated in FIG. 2.

[0035] In order to solve the problems as described above, a conventional steel (Comparative Example 3) in the related art may include a steel to which an inexpensive alloy specialized for forging may be added. The conventional steel may have reduced amounts of expensive alloy components, such as vanadium (V) and molybdenum (Mo), which may be required to secure the hardness at an unnecessary high temperature. In addition, the conventional steel may improve strength and toughness through the crystal grain refinement resulting from the addition of niobium (Nb) in order to secure abrasion resistance and impact toughness of the mold tool steel, and may secure hardness and abrasion resistance at normal temperature due to the increase in hardenability by adding chromium (Cr) and adding boron (B).

TABLE-US-00001 TABLE 1 Component (wt %) C Si Mn Ni Cr Mo V Nb B Comparative 0.32 to 0.80 to 0.50 or — 4.50 to 1.00 to 0.80 to — — Example 1 0.42 1.20 less 5.50 1.50 1.20 Comparative 0.49 to 0.15 to 0.70 to 1.50 to 0.70 to 0.20 to 0.10 to — — Example 2 0.54 0.35 1.00 1.80 1.20 0.40 0.15 Comparative 0.35 to 0.70 to 0.50 or — 6.00 to 0.70 to 0.40 to 0.10 to 0.001 to Example 3 0.45 0.90 less 8.00 0.90 0.60 0.20 0.002

[0036] Table 1 shows components in Comparative Example 1 to 3 according to the related art, and shows that iron (Fe) was included as a main component and components added are shown as weight % (wt %) based on the total weight of the entire mold tool steel.

[0037] In Comparative Example 3, the strength and hardness were in the same level as those in Comparative Example 2, when the same heat treatment as performed, and the strength and hardness were increased by 20% compared to Comparative Example 2. For example, about 14,000 strokes were achieved by a pair of molds used to produce an R engine con rod, and the manufacturing costs of parts were reduced. Further, Comparative Examples 1 to 3 were all in the same level in terms of impact toughness, but in the forging evaluation actually applied, the life in Comparative Examples 1 to 3 was in the same level as that of Comparative Example 2 and the life was improved by about 5 times or greater compared to Comparative Example 2. However, the conventional alloy steel composition such as Comparative Example 3 may have a problem in that the strength, hardness, and abrasion resistance required for a mold applied to mass production are not satisfied.

[0038] Accordingly, provided is an alloy steel for a mold too, which may include an amount of about 0.35 to 0.45 wt % of carbon (C), an amount of about 0.80 to 1.20 wt % of silicon (Si), an amount of about 0.20 to 0.50 wt % of manganese (Mn), an amount of about 6.00 to 8.00 wt % of chromium (Cr), an amount of about 1.50 to 3.00 wt % of molybdenum (Mo), an amount of about 0.80 to 1.20 wt % of vanadium (V), an amount of about 0.05 to 0.10 wt % of niobium (Nb), an amount of about 0.10 to 1.00 wt % of tungsten (W), and iron (Fe) constituting the remaining balance of the alloy steel, and all the wt % are based on the total weight of the alloy steel.

[0039] The process for the alloy steel composition of the present invention may include a forging process, and the maximum temperature of a mold for hot forging used at the highest temperature in the forging process may be of about 500° C. Accordingly, the content of alloy elements for improving physical properties at high temperature of about 500° C. may be optimized. By increasing the content of molybdenum (Mo) and chromium (Cr) compared to the related art, the strength and temper softening resistance at high temperature may be improved. Further, tungsten may be added to improve hardness at a high temperature due to the precipitation of fine tungsten (W) carbide (carbide), and the strength and toughness may be simultaneously improved through crystal grain refinement by adding niobium (Nb).

[0040] Hereinafter, the effects due to the addition of each alloy element of the present invention will be specifically examined.

[0041] (1) Carbon (C)

[0042] The carbon (C) as used herein may be an important element for securing the strength of an alloy steel, and may stabilize the residual austenite. For the role, the content of carbon (C) may be preferably about 0.35 to 0.45 wt % based on the total weight of the alloy steel.

[0043] Here, when the content of carbon (C) is less than about 0.35 wt %, the strength of the mold tool steel may not be sufficiently obtained, and the reduction in strength may be incurred, and the like. In contrast, when the content of carbon (C) is greater than about 0.45 wt %, undissolved large carbide may remain, thereby leading to reduction in strength and durability, and the like.

[0044] (2) Silicon (Si)

[0045] The silicon (Si) as used herein may be a deoxidizer, and may suppress formation of pinholes in a mold tool steel. The silicon component may be solid-solubilized in a matrix to increase the strength of an alloy steel by the solid solution strengthening effect, and enhance the activity of carbon (C), and the like. For the role, the content of silicon (Si) may be preferably about 0.80 to 1.20 wt % based on the total weight of the alloy steel.

[0046] When the content of silicon (Si) is less than about 0.80 wt %, oxides may remain in the alloy steel due to oxygen which may not be sufficiently removed, as consequence, the strength of the mold tool steel may be reduced, sufficient solid solution strengthening effects may not be obtained. When the content of silicon (Si) is greater than about 1.20 wt %, decarburization may be generated by an interpermeation reaction in the structure, such as a site competition reaction with carbon (C) due to the excessive content of silicon (Si).

[0047] (3) Manganese (Mn)

[0048] The manganese (Mn) as used herein may improve the hardenability of a mold tool steel, and enhance the strength of the mold tool steel, and the like. Preferably, the content of manganese (Mn) may be about 0.20 to 0.50 wt % based on the total weight of the alloy steel.

[0049] When the content of manganese (Mn) is less than about 0.2 wt %, an effect of improving the hardenability of a mold tool steel may not be sufficient. In contrast, when the content of manganese (Mn) is greater than about 0.50 wt %, the processability and the life may deteriorate.

[0050] (4) Chromium (Cr)

[0051] The chromium (Cr) as used herein may improve the hardenability of a mold tool steel, imparts curability, and refine structure of the alloy steel. Further, the chromium (Cr) may enhance the strength at high temperature, and enhance the temper softening resistance. Preferably, the content of chromium (Cr) may about 6.00 to 8.00 wt % based on the total weight of the alloy steel.

[0052] When the content of chromium (Cr) is less than about 6.00 wt %, the hardenability and curability may be limited, and sufficient structure refinement and spheroidization may not be obtained. When the content of chromium (Cr) is greater than about 8.00 wt %, the effect caused by an increase in content may not be sufficient, and thus, an increase in the manufacturing costs may be incurred.

[0053] (5) Vanadium (V)

[0054] The vanadium (V) as used herein may form a precipitate such as carbide, and strengthen the matrix structure thereby enhancing strength and abrasion resistance through the precipitation strengthening effect, and reducing the activity of carbon. Further, the strength of the alloy steel may be increased at the same cooling rate. Preferably, the content of vanadium (V) may be about 0.80 wt % to 1.20 wt % based on the total weight of the alloy steel.

[0055] Here, when the content of vanadium (V) is less than 0.80 wt % or greater than about 1.20 wt %, toughness and hardness of an alloy steel, and the like may be reduced.

[0056] (6) Molybdenum (Mo)

[0057] The molybdenum (Mo) as used herein may enhance the strength at high temperature by precipitation of molybdenum (Mo) carbide and increase the temper softening resistance. FIG. 3 shows an enlarged photograph illustrating molybdenum carbide of an exemplary alloy steel according to an exemplary embodiment of the present invention. As indicated by the arrow inside FIG. 3, molybdenum (Mo) carbide may be precipitated. Preferably, the content of molybdenum (Mo) may be about 1.50 to 3.0 wt % based on the total weight of the alloy steel.

[0058] When the content of molybdenum (Mo) is less than about 1.50 wt %, a sufficient strength may not be secured. When the content of molybdenum (Mo) is greater than about 3.0 wt %, the effects of yield strength and tensile strength may be reduced, and the effects due to an increase in content may not be sufficient, thereby incurring an increase in the manufacturing costs.

[0059] When this is further reviewed, FIG. 4 is a graph illustrating the yield strength according to an exemplary content of molybdenum of an exemplary alloy steel according to an exemplary embodiment of the present invention. In the graph of FIG. 4, the horizontal axis indicates the content of molybdenum and the unit is wt %, and the vertical axis indicates the yield strength and the unit is MPa. As illustrated in FIG. 4, strength may be substantially increased when the content of molybdenum is about 1.0 to 1.5 wt %. However, the increase rate in strength is rapidly decreased when the content of molybdenum is about 3.0 to 3.5 wt %.

[0060] Further, FIG. 5 is a graph illustrating the tensile strength according to an exemplary content of molybdenum of an exemplary alloy steel according to an exemplary embodiment of the present invention. In the graph of FIG. 5, the horizontal axis indicates the content of molybdenum and the unit is wt %, and the vertical axis indicates the tensile strength and the unit is MPa. As illustrated in FIG. 5, strength may be substantially increased when the content of molybdenum is about 1.0 to 1.5 wt %. However, the increase rate in strength may be rapidly decreased when the content of molybdenum is about 3.0 to 3.5 wt %.

TABLE-US-00002 TABLE 2 Content of Mo (wt %) Tensile strength (MPa) 1.0 520 1.5 610 2.0 670 2.5 700 3.0 700 3.5 710

[0061] In Table 2, the tensile strength was measured by changing only the content of molybdenum while the components of the present invention were the same as each other. As shown in Table 2, when molybdenum was added in an amount of 1.0 to 1.5 wt % based on the total weight of the alloy steel, the increase rate was 90 MPa, which was a rapidly increased value. When molybdenum was added in an amount of about 3.0 to 3.5 wt % based on the total weight of the alloy steel, the increase rate was about 10 MPa, which is a small increasedvalue.

[0062] (7) Tungsten (W)

[0063] The tungsten as used herein may enhance hardness, abrasion resistance, and toughness at high temperature because tungsten carbide may be precipitated. Preferably, the content of tungsten may be about 0.1 wt % to about 1.0 wt % based on the total weight of the alloy steel.

[0064] When the content of tungsten is less than about 0.10 wt %, hardness may not be sufficiently improved because tungsten carbide is not sufficiently precipitated. When the content of tungsten is greater than about 1.00 wt %, impact toughness may be reduced by precipitation of coarse tungsten carbide (carbide).

[0065] FIG. 6 is a graph illustrating the hardness according to the austenizing temperature in an alloy steel according to Comparative Example 1. In FIG. 6, the horizontal axis indicates the austenizing temperature and the unit is ° C., and the vertical axis indicates the hardness and the unit corresponds to HRC. In FIG. 6, when tungsten was added to Comparative Example 1, the hardness according to the austenizing temperature was increased as compared to that of Comparative Example 1 where tungsten was not added. FIG. 7 is an enlarged photograph illustrating tungsten carbide of an exemplary alloy steel according to an exemplary embodiment of the present invention. When the temperature is equal to or greater than the austenizing temperature, tungsten carbide may be precipitated, and as indicated by the arrow in FIG. 7, the precipitated tungsten carbide may be confirmed. Accordingly, the tungsten carbide in the alloy steel of the present invention may increase the hot abrasion resistance.

[0066] However, when the content of tungsten is greater than about 1.0 wt %, impact toughness is lowered by precipitation of coarse tungsten carbide. FIG. 8 is a graph illustrating the toughness according to an exemplary content of tungsten of an exemplary alloy steel according to an exemplary embodiment of the present invention. In FIG. 8, the horizontal axis indicates the content of tungsten and the unit is wt %, and the vertical axis indicates the toughness and the unit corresponds to J. As illustrated in FIG. 8, when the content of tungsten was about 0.1 wt % to 1.0 wt %, tungsten carbide was stabilized. When the content was greater than about 1.0 wt %, that carbide (carbide) may be coarsened by tungsten, and impact toughness may be reduced.

[0067] (8) Niobium (Nb)

[0068] When niobium is added, the toughness may be prevented from being reduced due to the refinement of crystal grains, and the corrosion fatigue life of the material may be enhanced. This may be proved by the following Equation 1.

σ.sub.0=σ.sub.i+K′d.sup.−1/2  [Equation 1] [0069] σ=Yield stress toughness [0070] σ=Dislocation motion obstruction frictional coefficient [0071] K′=Barrier integration constant of dislocation [0072] d=Diameter of crystal grains

[0073] According to Equation 1 (Hall-Petch equation), as the diameter of crystal grains is decreased, strength and toughness are increased. FIG. 9 is a schematic view illustrating the progression of cracks according to the size of large crystal grains, and FIG. 10 is a schematic view illustrating the progression of cracks according to the size of small crystal grains. When external force acts from the left side to the right side in FIGS. 9 and 10, the arrow indicates that cracks progress. Accordingly, the refiner the crystal grains are, the more the number of steps of progressing cracks is, and the corrosion fatigue life may be increased because it is difficult for cracks to progress.

TABLE-US-00003 TABLE 3 Content of Nb (wt %) Strength (MPa) Stretching ratio (%) 0.02 1,793 10.3 0.04 1,788 11.7 0.06 1,802 14.9 0.09 1,814 14.8 0.12 1,831 15.0

[0074] Table 3 shows an effect of enhancing the strength and the stretching ratio according to the amount of niobium component added in the components of the present invention. In Table 3, there may be a difference in stretching ratio according to the amount of Nb added.

[0075] When niobium was added in an amount of 0.02 wt %, the strength was 1,793 MPa and the stretching ratio was 10.3%. Further, when niobium was added in an amount of about 0.04 wt %, the strength was 1,788 MPa and the stretching ratio was 11.7%. Furthermore, when niobium was added in an amount of 0.06 wt %, the strength was 1,802 MPa and the stretching ratio was 14.9%. In addition, when niobium was added in an amount of 0.09 wt %, the strength was 1,814 MPa and the stretching ratio was 14.8%. Finally, when niobium was added in an amount of 0.12 wt %, the strength was 1,831 MPa and the stretching ratio was 15.0%. FIG. 11 is a graph illustrating the stretching ratio according to an exemplary content of niobium of an exemplary alloy steel according to an exemplary embodiment of the present invention. In FIG. 11, the horizontal axis of the graph indicates the content of niobium and the unit is wt %, and the vertical axis indicates the stretching ratio and the unit is %. As illustrated in FIG. 11, the stretching ratio may be rapidly increased from the point where the content of niobium is about 0.5 wt % based on the total weight of the alloy steel.

[0076] Accordingly, the content of niobium of the present invention preferably may be about 0.05 to 0.10 wt % based on the total weight of the alloy steel. When the content of niobium is less than about 0.05 wt %, the life may be shortened, and when the content of niobium is greater than about 0.10 wt %, the effect of enhancing the stretching ratio may be minimal, and thus the manufacturing costs may be increased.

[0077] In another aspect, the present invention relates to a mold including the alloy steel as described herein.

[0078] The mold may have substantially improved physical properties at high temperature, and may have an effect in that the life of the mold, which may be enhanced by about 40%. For example, 20,000 strokes may be used when the mold of the present invention is applied to the actual forging process whereas only 14,000 strokes may be used in the related art.

[0079] according to various exemplary embodiments, by adding molybdenum, tungsten, and niobium, strength at high temperature and temper softening resistance of the alloy steel may be increased, hot abrasion resistance thereof may be enhanced, and strength and toughness thereof also may be increased. Furthermore, by manufacturing a mold including the alloy steel of the present invention, the life of the mold may be increased.

[0080] As described above, the present invention has been described in relation to exemplary embodiments of the present invention, but the exemplary embodiments are only illustration and the present invention is not limited thereto. Exemplary embodiments described may be changed or modified by those skilled in the art to which the present invention pertains without departing from the scope of the present invention, and various alterations and modifications are possible within the technical spirit of the present invention and the equivalent scope of the claims which will be described below.