Disclosure of Invention
The invention aims to provide a tin-chromium composite plating steel plate and a preparation method thereof.
In order to achieve the above object, the present invention provides a method for preparing a tin-chromium composite plated steel sheet, the method comprising:
flattening the annealed steel plate to obtain a steel substrate with the roughness of 0.2-0.4 mu m;
pre-electroplating and tin electroplating are carried out on the steel substrate with the roughness of 0.2-0.4 mu m to obtain a tin-plated steel plate; the pre-electroplating solution adopted by the pre-electroplating comprises methanesulfonic acid and stannous methanesulfonate, and the electroplating tin adopts a methanesulfonic acid electroplating solution;
passivating and oiling the tin-plated steel plate to obtain a tin-chromium composite plated steel plate; wherein the passivating comprises: carrying out first passivation by adopting a first passivation solution, and then carrying out second passivation by adopting a second passivation solution, wherein the first passivation solution comprises the following final concentration components: 100-150g/L CrO30.4-0.6g/L of H2SO44-6g/L of Na2FSi6The second passivation solution comprises the following components in final concentration: 50-80g/L CrO30.05-0.15g/L of H2SO40.04-0.06g/L of Na2FSi6。
Further, the leveling adopts a double-frame wet leveling mode, the double-frame comprises a first frame adopting a texturing roller with the roughness of 1.1-1.3 μm and a second frame adopting a grinding roller with the roughness of 0.3-0.5 μm, and the distribution ratio of the stretch ratio of the straightening and the leveling is (0.8-1.2): (0.8-1.2).
Further, the pre-plating solution comprises the following components in final concentration: 4-6g/L methanesulfonic acid, 5-15mgSn of/L2+。
Further, the immersion cleaning temperature of the pre-electroplating is 35-45 ℃.
Further, the methane sulfonic acid plating solution comprises the following components in final concentration: 20-30g/L Sn2+30-50ml/L of methanesulfonic acid, 10-20ml/L of antioxidant and 2-5ml/L of additive, wherein the additive comprises at least one of polyoxyethylene, polyethylene glycol and sodium dodecyl benzene sulfonate.
Further, the current density of the electrolytic tin plating is 5-10A/dm2The temperature of the electrolytic tinning is 35-45 ℃.
Further, the temperature of the first passivation is 40-45 ℃, and the current density of the first passivation is 55-65A/dm2。
Further, the temperature of the second passivation is 40-45 ℃, and the current density of the second passivation is 15-25A/dm2。
The invention also provides a tin-chromium composite coating steel plate, which comprises the following components:
a steel substrate having a roughness of 0.2-0.4 μm;
the tin-chromium composite coating is coated on the surface of the steel substrate and comprises a metal chromium layer, a metal tin layer and a hydrated chromium oxide layer, wherein each m of the tin-chromium composite coating is2The mass of the metal tin in the steel substrate is 0.1-0.8g, the mass of the metal chromium is 30-50mg, and the mass of the hydrated chromium oxide is 5-12 mg; the metallic tin layer is in an island shape with the diameter of 0.2-1.5 mu m.
Further, the chemical composition of the steel substrate comprises the following components in percentage by mass: c: 0.02 to 0.06%, Si: 0.01-0.02%, Mn: 0.1-0.5%, P: 0.012-0.015%, S: 0.008-0.015%, Als: 0.025 to 0.085 percent of Cu, less than or equal to 0.2 percent of Ni, less than or equal to 0.15 percent of Cr, less than or equal to 0.1 percent of Mo, less than or equal to 0.0035 percent of N, less than or equal to 0.003 percent of O, and the balance of Fe and inevitable impurities.
One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:
the invention providesThe tin-chromium composite plating steel plate and the preparation method thereof comprise the following steps: flattening the annealed steel plate to obtain a steel substrate with the roughness of 0.2-0.4 mu m; pre-electroplating and tin electroplating are carried out on the steel substrate with the roughness of 0.2-0.4 mu m to obtain a tin-plated steel plate; the pre-electroplating solution adopted by the pre-electroplating comprises methanesulfonic acid and stannous methanesulfonate, and the electroplating tin adopts a methanesulfonic acid electroplating solution; passivating and oiling the tin-plated steel plate to obtain a tin-chromium composite plated steel plate; wherein the passivating comprises: carrying out first passivation by adopting a first passivation solution, and then carrying out second passivation by adopting a second passivation solution, wherein the first passivation solution comprises the following final concentration components: 100-150g/L CrO30.4-0.6g/L of H2SO44-6g/L of Na2FSi6The second passivation solution comprises the following components in final concentration: 50-80g/L CrO30.05-0.15g/L of H2SO40.04-0.06g/L of Na2FSi6. The embodiment of the invention controls the roughness of the steel substrate to be 0.2-0.4 μm, which is beneficial to the distribution uniformity of the composite plating and improves the corrosion resistance of the product; the methanesulfonic acid and the stannous methanesulfonate are innovatively adopted as the pre-electroplating solution, so that the action of activating the strip steel is achieved, and island-shaped deposition of a tin layer is facilitated; the tin electroplating process adopts environment-friendly MSA methane sulfonic acid electroplating solution to obtain island-shaped tin with the diameter of 0.2-1.5 mu m, and greatly improves the welding performance of the composite plating plate; the two-step passivation method is favorable for accurately controlling the contents of the simple substance chromium layer and the chromium oxide layer, so that the coating performance is improved, and the tin-chromium composite coating steel plate has good corrosion resistance, good welding performance and good coating performance.
Detailed Description
The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that these specific embodiments and examples are for the purpose of illustrating the invention and are not to be construed as limiting the invention.
Throughout the specification, unless otherwise specifically noted, terms used herein should be understood as having meanings as commonly used in the art. Accordingly, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. If there is a conflict, the present specification will control.
Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.
In order to solve the technical problems, the embodiment of the invention provides the following general ideas:
according to an exemplary embodiment provided by an embodiment of the present invention, there is provided a method for manufacturing a tin-chromium composite plated steel sheet, as shown in fig. 1, the method including:
s1, flattening the annealed steel plate to obtain a steel substrate with the roughness of 0.2-0.4 μm;
the roughness of the steel substrate is 0.2-0.4 μm, which is beneficial to the distribution uniformity of the composite coating and improves the corrosion resistance of the product; if the roughness of the steel substrate is less than 0.2 mu m, the adhesive force performance of the composite coating product in downstream coating and printing use is adversely affected; if the roughness of the steel substrate is more than 0.4 mu m, the uniform distribution of island-shaped tin is not facilitated, and the welding performance and the corrosion resistance are influenced; the purpose of the roll matching mode and the elongation rate distribution of the wet flattening process is to control the roughness of a steel substrate to be 0.2-0.4 mu m;
as an alternative embodiment, the leveling is a double-stand wet leveling method, the double stand comprises a stand using a roughening roller with a roughness of 1.1-1.3 μm and a stand using a grinding roller with a roughness of 0.3-0.5 μm, and the distribution ratio of the elongation ratio of the straightening and the leveling is (0.8-1.2): (0.8-1.2). The roller distribution mode and the elongation rate distribution of the double-frame wet and flat are favorable for the distribution uniformity of the composite coating and the corrosion resistance of the product is improved;
if the roughness of the roughened roller is less than 1.1 mu m, the roughness of the steel substrate is less than 0.2 mu m; if it is more than 1.3 μm, it results in a steel substrate roughness of more than 0.4. mu.m; the roughness of the texturing roller is preferably 1.2 μm;
if the roughness of the grinding roller is less than 0.3 mu m, the roughness of the steel substrate is more than 0.4 mu m; if it is more than 0.5. mu.m, it results in a steel substrate roughness of less than 0.2. mu.m; the roughness of the grinding roller is preferably 0.4 μm;
if the elongation ratio distribution ratio is less than 0.8: 1.2, resulting in a steel substrate roughness below 0.2 μm; if the ratio is more than 1.2: 0.8, resulting in a steel substrate roughness greater than 0.4 μm; the elongation ratio distribution ratio is preferably 1: 1;
s2, pre-electroplating and tin electroplating are carried out on the steel substrate with the roughness of 0.2-0.4 μm to obtain a tin-plated steel plate; the pre-electroplating solution adopted by the pre-electroplating comprises methanesulfonic acid and stannous methanesulfonate, and the electroplating tin adopts a methanesulfonic acid electroplating solution;
according to the embodiment of the invention, methanesulfonic acid and stannous methanesulfonate are innovatively adopted as the pre-electroplating solution, so that the action of activating the strip steel is achieved, nucleation sites during tin electroplating are provided, and island-shaped deposition of a tin layer is facilitated; in particular, the amount of the solvent to be used,
the pre-plating solution comprises the following components in final concentration: 4-6g/L of methanesulfonic acid, 5-15mg/L of Sn2+. The methanesulfonic acid is used for removing an oxide layer on the surface of the strip steel, if the concentration of the methanesulfonic acid is too low, the oxide layer is not completely removed, the deposition of a tin layer is influenced, and if the concentration of the methanesulfonic acid is too high, the adverse influence of damaging the surface of the substrate is caused; sn (tin)2+The function of the tin-plating solution is to enrich the surface of the activated strip steel and increase nucleation sites when tin is electroplated, if Sn is2+If the concentration is too low, island-like deposition of tin layer is not favored in the tin electroplating stage, if Sn is2+Too high a concentration may inhibit the influence of methanesulfonic acid on the activation of the strip.
The pre-plating process is only immersion cleaning and is not electrified, and the temperature of the pre-plating process is 35-45 ℃. The pre-electroplating condition is favorable for increasing nucleation sites during tin electroplating and is favorable for island formation and distribution uniformity of tin layer deposition.
The tin electroplating process of the embodiment of the invention adopts environment-friendly MSA methane sulfonic acid electroplating solution to obtain island-shaped tin with the diameter of 0.2-1.5 mu m, thereby greatly improving the welding performance of the composite plating plate; in particular, the amount of the solvent to be used,
the methane sulfonic acid electroplating solution comprises the following components in final concentration: 20-30g/L Sn2+30-50ml/L of methanesulfonic acid, 10-20ml/L of antioxidant and 2-5ml/L of additive, wherein the additive comprises at least one of polyoxyethylene, polyethylene glycol and sodium dodecyl benzene sulfonate. Sn (tin)2+Has the function of providing a primary salt for cathodic electrodeposition, if Sn2+Too low concentration, fine crystal, not good for island-like tin formation, if Sn2+The over-high concentration causes the reduction of the dispersion capability of the plating solution, aggravates the edge effect and is not beneficial to the uniform distribution of the plating layer; the additive has the functions of increasing cathode polarization and refining crystal grains, hydrogen evolution is easy to occur if the concentration of the additive is too low, and island-shaped tin cannot be formed if the concentration of the additive is too high;
the current density of the electrolytic tin plating is 5-10A/dm2The temperature of the electrolytic tinning is 35-45 ℃.
The embodiment of the invention applies 2-5ml/L low-concentration additive and 5-10A/dm2The current density is low, so that the size of tin crystal grains is coarsened, island-shaped tin with the size of 0.2-1.5 mu m can be obtained, and the welding performance of the composite plating plate is greatly improved;
s3, passivating and oiling the tin-plated steel plate to obtain a tin-chromium composite plated steel plate; wherein the passivating comprises: carrying out first passivation by adopting a first passivation solution, and then carrying out second passivation by adopting a second passivation solutionThe first passivation solution comprises the following components in final concentration: 100-150g/L CrO30.4-0.6g/L of H2SO44-6g/L of Na2FSi6The second passivation solution comprises the following components in final concentration: 50-80g/L CrO30.05-0.15g/L of H2SO40.04-0.06g/L of Na2FSi6。
The adoption of a two-step passivation method is favorable for accurately controlling the contents of the simple substance chromium layer and the chromium oxide layer to obtain the optimal metal chromium layer and the optimal chromium oxide layer, and particularly, every m can be ensured2The mass of the metal tin in the steel substrate is 0.1-0.8g, the mass of the metal chromium is 30-50mg, and the mass of the hydrated chromium oxide is 5-12mg, so that the finishing performance is improved.
CrO in the second passivation solution of the embodiment of the invention3,H2SO4,Na2FSi6The concentrations of the three components are all lower than the concentrations of the three components in the first passivation solution, for the reasons that: the high-concentration passivation solution is matched with high current density to form simple substance chromium, and the low-concentration passivation solution is matched with low current density to form chromium oxide layer;
according to another exemplary embodiment of the embodiments of the present invention, there is provided a tin-chromium composite plated steel sheet, as shown in fig. 2, including:
a steel substrate 1, wherein the roughness of the steel substrate is 0.2-0.4 μm;
the tin-chromium composite coating 2 is coated on the surface of the steel substrate and comprises a metal chromium layer 21, a metal tin layer 22 and a hydrated chromium oxide layer 23, wherein each m of the tin-chromium composite coating is2The mass of the metal tin in the steel substrate is 0.1-0.8g, the mass of the metal chromium is 30-50mg, and the mass of the hydrated chromium oxide is 5-12 mg; the metallic tin layer is in an island shape with the diameter of 0.2-1.5 mu m.
The surface of the hydrated chromium oxide layer 23 is also covered with an oil film 24.
The embodiment of the invention controls the roughness of the steel substrate to be 0.2-0.4 μm, which is beneficial to the distribution uniformity of the composite plating and improves the corrosion resistance of the product; diameter ofIsland-shaped tin with the thickness of 0.2-1.5 mu m is adopted, so that the welding performance of the composite plating plate is greatly improved; "per m2The mass of the metal tin in the steel substrate is 0.1-0.8g, the mass of the metal chromium is 30-50mg, the mass of the hydrated chromium oxide is 5-12mg, and the contents of a simple substance chromium layer and a chromium oxide layer are precisely controlled to obtain the optimal metal chromium layer and the optimal chromium oxide layer, so that the coating performance is improved.
The tin-chromium composite plated steel plate provided by the embodiment of the invention can be prepared by the preparation method of the tin-chromium composite plated steel plate provided by the embodiment of the invention, but other methods which can be developed in the future can be adopted, as long as the method can ensure that the roughness of a steel substrate is 0.2-0.4 mu m, and a metal tin layer in the tin-chromium composite plated layer is in an island shape with the diameter of 0.2-1.5 mu m, and each m is an island shape2The mass of the metal tin in the steel substrate is 0.1-0.8g, the mass of the metal chromium is 30-50mg, and the mass of the hydrated chromium oxide is 5-12 mg; therefore, the tin-chromium composite plating steel plate has good corrosion resistance, good welding performance and good finishing performance.
The tin-chromium composite coating steel plate provided by the embodiment of the invention is mainly used for food packaging and can also be used for chemical barrels or spray cans.
As an alternative embodiment, the steel substrate has the following chemical composition in mass fraction: ranges of common chemical compositions for steel substrates for food packaging, for example: c: 0.02 to 0.06%, Si: 0.01-0.02%, Mn: 0.1-0.5%, P: 0.012-0.015%, S: 0.008-0.015%, Als: 0.025 to 0.085 percent of Cu, less than or equal to 0.2 percent of Ni, less than or equal to 0.15 percent of Cr, less than or equal to 0.1 percent of Mo, less than or equal to 0.0035 percent of N, less than or equal to 0.003 percent of O, and the balance of Fe and inevitable impurities.
A tin-chromium composite plated steel sheet and a method for manufacturing the same according to the present application will be described in detail with reference to examples, comparative examples, and experimental data.
Example 1
S1, adopting a one-time cold rolling continuous annealing plate T-4CA with the thickness specification of 0.20mm and the width specification of 832mm, and flattening the annealed steel plate after the annealing of the strip steel is finished to obtain a steel substrate with the roughness of 0.35 mu m; the leveling adopts a double-frame wet leveling process, one frame adopts a roughening roller, the roughness is 1.2 mu m, the two frames adopt grinding rollers, the roughness is 0.4 mu m, and the elongation percentage distribution is 1: 1;
step S2, pre-electroplating and electrotinning the steel substrate with the roughness of 0.35 mu m to obtain a tin-plated steel plate; wherein the pre-plating solution in the pre-plating process contains 4g/L of methanesulfonic acid, Sn2+The concentration is 5 mg/L; the tinning section adopts MSA electroplating solution, and the electroplating process condition is Sn2+The concentration is 20g/L, the methanesulfonic acid is 30ml/L, the additive is 2ml/L, the antioxidant is 10ml/L, the solution temperature is controlled at 35 ℃, and the current density is 5A/dm2;
Step S3, passivating and oiling the tin-plated steel plate to obtain a tin-chromium composite plated steel plate; wherein, the passivation adopts a two-step method, and the first passivation process conditions are as follows: passivating solution CrO3Concentration of 100g/L, passivation temperature of 40 ℃ and H2SO4Concentration 0.5g/L, Na2FSi6The concentration is 5g/L, and the current density is 60A/dm2And the second passivation process condition is as follows: passivating solution CrO3Concentration of 50g/L, passivation temperature of 40 ℃ and H2SO4Concentration 0.1g/L, Na2FSi6The concentration is 0.05g/L, and the current density is 20A/dm2。
The finally obtained Sn-Cr composite coating steel plate has the roughness of 0.35 mu m and the tin plating amount of 0.15g/m2Island size 0.3 μm, amount of metallic chromium 35mg/m2The amount of hydrated chromium oxide is 8mg/m2The salt spray grade is 1 grade, the adhesive force is 1 grade, the sulfur resistance is 1 grade, and the welding performance is good when the paint is applied to a protein beverage can.
Example 2
Step S1, adopting a one-time cold rolling continuous annealing plate T-5CA with the thickness specification of 0.20mm and the width specification of 870mm, and flattening the annealed steel plate after the annealing of the strip steel is finished to obtain a steel substrate with the roughness of 0.4 mu m; the leveling adopts a wet leveling process, one machine frame adopts a roughening roller, the roughness is 1.1 mu m, the other machine frame adopts a grinding roller, the roughness is 0.3 mu m, and the elongation percentage distribution is 0.8 during leveling: 1.2;
step S2, pre-electroplating and electrotinning the steel substrate with the roughness of 0.4 mu m to obtain a tin-plated steel plate; it is composed ofThe pre-plating solution in the middle pre-plating process contains 5g/L of methanesulfonic acid, Sn2+The concentration is 10 mg/L; the tinning section adopts MSA electroplating solution, and the electroplating process condition is Sn2+The concentration is 25g/L, the methanesulfonic acid is 40ml/L, the additive is 4ml/L, the antioxidant is 15ml/L, the solution temperature is controlled at 40 ℃, and the current density is 8A/dm2;
Step S3, passivating and oiling the tin-plated steel plate to obtain a tin-chromium composite plated steel plate; wherein, the passivation adopts a two-step method, and the first passivation process conditions are as follows: passivating solution CrO3Concentration of 120g/L, passivation temperature of 42 ℃, H2SO4Concentration 0.5g/L, Na2FSi6The concentration is 5g/L, and the current density is 60A/dm2And the second passivation process condition is as follows: passivating solution CrO3Concentration 65g/L, passivation temperature 42 ℃, H2SO4Concentration 0.1g/L, Na2FSi6The concentration is 0.05g/L, and the current density is 20A/dm2。
The finally obtained Sn-Cr composite coating steel plate has the roughness of 0.4 mu m and the tin plating amount of 0.6g/m2The tin layer is island-shaped, the size is 0.5 mu m, and the amount of the metal chromium is 35mg/m2The amount of hydrated chromium oxide is 10mg/m2 Salt spray grade 1, adhesive force grade 1 and sulfur resistance grade 1, and is applied to eight-treasure porridge pots and has good welding performance.
Example 3
Step S1, adopting a one-time cold rolling cover to retreat the T-3BA, wherein the thickness specification is 0.23mm, the width specification is 906mm, and flattening the annealed steel plate after the annealing of the strip steel is finished to obtain a steel substrate with the roughness of 0.2 mu m; the leveling adopts a wet leveling process, one machine frame adopts a roughening roller, the roughness is 1.3 mu m, the other machine frame adopts a grinding roller, the roughness is 0.5 mu m, and the elongation percentage distribution is 1.2 during leveling: 0.8;
step S2, pre-electroplating and electrotinning the steel substrate with the roughness of 0.2 mu m to obtain a tin-plated steel plate; wherein the pre-plating solution in the pre-plating process contains 6g/L of methylsulfonic acid, Sn2+The concentration is 15 mg/L; the tinning section adopts MSA electroplating solution, and the electroplating process condition is Sn2+The concentration is 30g/L, the methanesulfonic acid is 50ml/L, the additive is 5ml/L, the antioxidant is 20ml/L, the solution temperature is controlled at 45 ℃, and the electrolyte is chargedFlow density 10A/dm2;
Step S3, passivating and oiling the tin-plated steel plate to obtain a tin-chromium composite plated steel plate; wherein, the passivation adopts a two-step method, and the first passivation process conditions are as follows: passivating solution CrO3Concentration of 150g/L, passivation temperature of 45 ℃ and H2SO4Concentration 0.5g/L, Na2FSi6The concentration is 5g/L, and the current density is 60A/dm2And the second passivation process condition is as follows: passivating solution CrO3Concentration 80g/L, passivation temperature 45 ℃, H2SO4Concentration 0.1g/L, Na2FSi6The concentration is 0.05g/L, and the current density is 20A/dm2。
The Sn-Cr composite coating steel plate finally obtained has the roughness of 0.2 mu m, the tin coating amount of 0.6, the island-shaped tin layer with the size of 0.6 mu m and the metal chromium amount of 50mg/m2The amount of hydrated chromium oxide is 12mg/m2. The salt spray grade is 1 grade, the adhesive force is 1 grade, and the welding performance is good when the paint is applied to a milk powder tank.
Comparative example 1
In this comparative example, the roughness of the steel substrate in step S1 was 0.1. mu.m, and steps S2 and S3 were the same as in example 1.
Comparative example 2
In this comparative example, the roughness of the steel substrate in step S1 was 0.5. mu.m, and steps S2 and S3 were the same as in example 1.
Comparative example 3
The preplating solution used in step S2 of this comparative example was 2g/L H2SO4The electroplating solution adopted by the electrotinning is a common electroplating solution: 15g/L of Sn2+30-50ml/L of methanesulfonic acid, 10-20ml/L of antioxidant, 10-15ml/L of additive, step S1 and step S3 are the same as in example 1.
Comparative example 4
In the comparative example, the step S3 adopts one-step passivation, and the formula of the passivation solution is specifically 15-25g/L Na2Cr2O7Step S1 and step S2 are the same as in embodiment 1.
Experimental example 1
The properties of the tin-chromium composite plated steel sheets obtained in examples 1 to 3 and comparative examples 1 to 4 were measured and counted as shown in table 1.
TABLE 1
Note: the more the welding performance index "+" indicates the better the welding performance
As can be seen from the data in Table 1:
in comparative example 1, the roughness of the steel substrate was 0.1 μm, which is smaller than the range of 0.2 to 0.4 μm of the inventive example, and there was a disadvantage that the adhesion of the paint film was poor;
in comparative example 2, the steel substrate having a roughness of 0.5 μm, which is greater than the range of 0.2 to 0.4 μm of the inventive example, had a disadvantage of poor corrosion resistance;
in the comparative example 3, the pre-plating solution and the electrotinning solution are common plating solutions, and have the defect of poor welding performance;
in the comparative example 4, the one-step passivation method has the defects of poor corrosion resistance, poor paint film adhesion and poor sulfur resistance;
the tin-chromium composite plating steel plate prepared in the embodiment 1-3 has good corrosion resistance, good welding performance and good finishing performance.
Fig. 3 is an electron microscope image of island-shaped tin contained in a tin-chromium composite plated steel plate provided by the embodiment of the invention, which shows that island-shaped tin with a diameter of 0.2-1.5 μm is successfully obtained by the preparation method of the tin-chromium composite plated steel plate provided by the embodiment of the invention, and the welding performance of the composite plated plate is greatly improved;
finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.