A STEEL FOR GRADE R6 OFFSHORE MOORING CHAIN WITH HIGH STRENGTH AND HIGH TOUGHNESS AND ITS CHAIN USE IN ANCHORING AND MOORING FLOATING BODIES WITH CATHODIC PROTECTION

Abstract

The present application relates to a steel for grade R6 offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection: the chemical composition are C 0.18˜0.24%, N 0.006˜0.024, P 0.005˜0.025, S≤0.005, Si 0.15˜0.35, Mn 0.20˜0.40, Cr 1.40˜2.60, Ni 0.80˜3.20, Mo 0.35˜0.75, Cu≤0.50, Al≤0.02, Ti≤0.005, V 0.04˜0.12, Nb 0.02˜0.05, Ca 0.0005˜0.004, O≤0.0015, H≤0.00015, the balance is Fe: the total content of alloy ΣM=(Si+Mn+Cr+Ni+Mo+Cu), 3.4<ΣM≤6.8; the total content of microalloy ΣMM=(Ti+Al+Nb+V), 0.065≤ΣMM≤0.194. The corrosion potential is adjusted to prevent hydrogen embrittlement caused by cathodic overprotection on the basic premise of maintaining the strength, toughness and low corrosion rate of the steel. Where V is only used for strengthening, and the content of N in VCN is increased, especially for the increase of the temperature for chain quenching to make M3C, M2C and VCN fully dissolved in solid solution and fully precipitated in tempering, which improves the precipitation strengthening effect.

Claims

1. A steel for grade R6 offshore mooring chain with high strength and high toughness for use in anchoring and mooring floating bodies with cathodic protection, wherein the chemical composition by wt % (percentage by weight) are as follows: C 0.18˜0.24%, N 0.006˜0.024, P 0.005˜0.025, S≤0.005, Si 0.15˜0.35, Mn 0.20˜0.40, Cr 1.40˜2.60, Ni 0.80˜3.20, Mo 0.35˜0.75, Cu≤0.50, Al≤0.02, Ti≤0.005, V 0.04˜0.12, Nb 0.02˜0.05, Ca 0.0005˜0.004, O≤0.0015, H≤0.00015, the balance is Fe and unavoidable impurity elements; It is further defined that 0.22≤(C+N)≤0.26; the total content of alloy ΣM=(Si+Mn+Cr+Ni+Mo+Cu), 3.4≤ΣM≤6.8; the total content of micro-alloy ΣMM=(Ti+Al+Nb+V), 0.065≤ΣMM≤0.194.

2. The steel for grade R6 offshore mooring chain with high strength and high toughness for use in anchoring and mooring floating bodies with cathodic protection according to claim 1, wherein wt % of N in the chemical composition is 0.016-0.024.

3. A grade R6 offshore mooring chain with high strength and high toughness for use in anchoring and mooring floating bodies with cathodic protection, wherein the chemical composition by wt % (percentage by weight) are as follows: C 0.18˜0.24%, N 0.006˜0.024, P 0.005˜0.025, S≤0.005, Si 0.15˜0.35, Mn 0.20˜0.40, Cr 1.40˜2.60, Ni 0.80˜3.20, Mo 0.35˜0.75, Cu≤0.50, Al≤0.02, Ti≤0.005, V 0.04˜0.12, Nb 0.02˜0.05, Ca 0.0005˜0.004, O≤0.0015, H≤0.00015 the balance is Fe and unavoidable impurity elements: It is further defined that 0.22≤(C+N)≤0.26; the total content of alloy ΣM=(Si+Mn+Cr+Ni+Mo+Cu), 3.4≤ΣM≤6.8; the total content of micro-alloy ΣMM=(Ti+Al+Nb+V), 0.065≤ΣMM≤0.194.

4. The grade R6 offshore mooring chain with high strength and high toughness for use in anchoring and mooring floating bodies with cathodic protection according to claim 3, wherein wt % of N in the chemical composition is from 0.016 to 0.024.

5. The grade R6 high strength and high toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 3, wherein: with the combination and limitation of alloy element, composite bainite is transformed during the cooling process after the chain is austenitized, wherein the composite bainite is composed of upper bainite (BU), a small amount of lower bainite (BL) and martensite (M), and the microstructure docs not include granular bainite or ferrite; at the position that has a distance that is about a third of radius from the surface of the chain, the volume fraction of BL+M is no more than 10% and the grain size of the prior austenite is between grade 7.5 and grade 9.0.

6. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 3, wherein: with the combination and limited content of micro-alloyed elements MM and the limited content of C+N, the chain microstructure contains the precipitated extremely fine MCN type carbonitride with an average size of 2 nm, the carbonitride is VMoCN or VCN because its main composition of M(metal elements) is V, is written as VCN.

7. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 6, wherein: with the combination and limited content of microalloyed elements MM and the limited content of C+N, the N content of MCN type carbonitride is significantly increased wherein nearly half of the total amount of V corresponds to a chemical equivalent ratio of V:N=3.6 in the VCN precipitated, when calculated in terms of chemical equivalent ratio of Ti:N=3.4, Al:N=2:1, Nb:N=6.6, and V:N=3.6.

8. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring cathodic floating bodies with protection according to claim 3, wherein: the flat specimen of the chain is sampled and immersed in the artificial seawater prepared in accordance with ASTM d1141, and after immersion at the room temperature of 25° C. for 80 hours, the laboratory stable corrosion potential is measured to be about −610 to −650 MV (SCE).

9. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 8, wherein: according to the standards of DNVGL, SSRT is carried out in artificial seawater under the condition of without electric potential, with the electric potential of −850 mV, −1200 mV (SCE), respectively, and the strain rate of cylindrical smooth specimen is ≤10.sup.−5/s; Z.sub.0 and Z.sub.E refer to the reduction of area without electric potential and with the electric potential of −850 mV or −1200 mV (SCE) respectively; when the applied potentials are −850 mV and −1200 mV (SCE), the value of Z.sub.e/Z.sub.0 are 1 and ≤0.18 respectively, that is, no embrittlement and serious embrittlement.

10. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 8, wherein: according to the standards of DNVGL, the compact tensile (CT) test with the tension speed of ≤6*10.sup.−9 m/s is carried out in artificial seawater under the condition of without electric potential, with the electric potential of −950, −1050 mv (SCE), respectively; K.sub.QEAC.sub.0, and K.sub.QEAC.sub.E represent the results of the compact tensile test without electric potential and with electric potential respectively; when the CT specimen with precracking was precharged with hydrogen for 48 hours, the results of K.sub.QEAC.sub.0 and K.sub.QEAC.sub.E with applied potential of −1050 mV are met plane strain conditions, the criteria of KIC is satisfied, and the value of K.sub.IEAC.sub.E/K.sub.IEAC.sub.0 is 0.85; with an applied potential of −950 mV, the value of K.sub.QEAC.sub.0/K.sub.QEAC.sub.E of the welding line is 0.88; When K.sub.QEAC.sub.0/K.sub.QEAC.sub.E≥0.80, the reduction of EAC resistance is within the controllable range, and the values of K.sub.IC and K.sub.Q of the chain are higher.

11. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 3, wherein; the chain is made of round bar having corresponding chemical composition, and the round bar is processed in sequence by chain making, flash butt welding, and heat treatment to obtain the final product wherein the heat treatment include high-temperature quenching and tempering, with the high-temperature quenching temperature of ≥980° C. water quenching with the water temperature of lower than 50° C.; and the tempering temperature is from 600° C. to 690° C. water cooling with the water temperature is lower than 50° C.

12. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 10, wherein; the round bar is made from continuous casting bloom or ingot having corresponding chemical composition, which is processed in sequence by heating, blooming, rolling and slow cooling, in which the heating temperature is more than 1230° C. so that nitride and carbonitride are all dissolved in austenite; in the cooling process, due to the combination of microalloying elements and the limited content of C+N, the precipitating sequence of nitride and carbonitride is TiN—AlN—NbCN-MCN.

13. The grade R6 high-strength and high-toughness offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection according to claim 1, which is also suitable for the production of long and flat structural steel with high strength and toughness.

14. The grade R6 high-strength and high-toughness offshore mooring chain steel for use in anchoring and mooring floating bodies with cathodic protection according to claim 1, which is also suitable for the production of long and flat structural steel with high strength and toughness and with the resistance to deterioration of seawater environmental service performance.

Description

DESCRIPTION OF FIGURES

[0061] FIG. 1 The optical microscopy structure of carrying out example 2 of the invention, wherein the quenching structure is BU+BL+M, and (BL+M)≤10%;

[0062] FIG. 2 CCT schematic curve of the present invention

[0063] FIG. 3 The optical microscopy structure with grain size number 9 (80%) to 7.5 (20%) original fine-grained austenite, which is shown is rolled once and quenched at 980° C. after the blooming:

[0064] The size of the effective grain for toughening is smaller because of the existence of substructure;

[0065] FIG. 4 The distribution of C, Cr, Mo and V atoms in type M2C and MC carbide, precipitated during quenching-tempering of the steel of the present invention, measured by three-dimensional atomic probe;

[0066] FIG. 5 The sampling schematic of CT test and z-x direction specimen is adopted in the invention;

[0067] FIG. 6 The sire of CT specimen according to DNVGL.

[0068] FIG. 7 SSRT test by applied potential

[0069] FIG. 8 CT test by applied potential

MODE(S) FOR CARRYING OUT THE INVENTION

[0070] The present invention is further described in detail with reference to carrying out example.

[0071] Carrying out example (invention example) 1-4 and contrast example 3 are about the process wherein continuous casting bloom with tire size of 390×510 mm are rolled into the round bars with a diameter of 120 mm, while contrast example 1, 2 and 4 are about the process that 420 kg test ingots are forged into round bars with a diameter of 95 mm, and then the round bars are processed in sequence by blanking, heating, bending, flash welding, forming chain and heat treatment (quenching+tempering) to obtain the finished chain. The performance data is the average value of the results of three groups of specimens. Numerical treatment of the third place after the decimal point: ≤discard and ≥6 into 1.

[0072] See Table 2 for the chemical composition of carrying out example 1 to 4 and contrast examples 1 to 4. See Table 3 for process parameters and performance of the chain, and see Table 4 for the size and test results of CT specimens. Some of the results of Table 4 have been collated and included in Table 3.

TABLE-US-00002 TABLE 2 The chemical composition wt, % of invention example 1 to 4 and contrast examples 1 to 4 in the present invention; the thermodynamic software estimation of the precipitation temperature of nitride(MN), carbonilride(MCN), carbide (MC) Example C Si Mn Cr Ni Mo Cn ΣM Al Nb V Ti ΣMM N Invention 1 0.21 0.28 0.27 1.4 1.1 0.5 0.21 3.76 0.018 0.04 0.06 0.002 0.12 0.019 Invention 2 0.22 0.34 0.38 1.75 2.1 0.48 0.25 5.30 0.016 0.036 0.047 0.002 0.101 0.020 Invention 3 0.23 0.32 0.34 1.95 3.1 0.68 0.4 6.79 0.019 0.05 0.12 0.003 0.192 0.023 Invention 4 0.22 0.17 0.23 1.45 0.84 0.49 0.28 3.46 0.01 0.022 0.06 0.001 0.093 0.018 Contrast 1 0.28 0.33 0.35 1.2 2.6 0.7 0.17 5.35 0.02 0.05 0.09 0.005 0.185 0.013 Contrast 2 0.23 0.25 0.29 1.8 0.8 0.39 0.15 3.68 0.019 0.048 0.07 0.002 0.139 0.005 Contrast 3 0.23 0.25 0.35 2.55 3.05 0.65 0.45 7.30 0.017 0.07 0.10 0.003 0.190 0.023 Contrast 4 0.24 0.25 0.35 2.10 0.95 0.49 0.11 4.25 0.05 0.05 0.09 0.006 0.196 0.018 Evaluated precipitation temperature of MN and MCN,° C. Precipitation sequence of MN or MC, Example TiN NbCN VCN AlN MCN in the cooling process Invention 1 1439 1079 939 1201 TiN—AlN—NbCN—VCN Invention 2 1434 1102 928 1216 TiN—AlN—NbCN—VCN Invention 3 1434 1105 965 1193 TiN—AlN—NbCN—VCN Invention 4 1432 1104 949 1186 TiN—AlN—NbCN—VCN Contrast 1 1430 1117 955 1209 TiN—AlN—NbC—VC Contrast 2 1332 1211 860 947 TlN—NbCN—AlN—VC Contrast 3 1426 1207 948 1195 TiN—NbCN—AlN—VCN Contrast 4 1442 1114 949 1248 TiN—AlN—NbC—VC S ≤ 0.002 P ≤ 0.010

TABLE-US-00003 TABLE 3 Process parameters and performance of invention example of d120 mm grade R6 steel and chain, as well as performance of their contrast examples Quenching temperature Example C + N ΣM ΣMM of link ~Ms ~Bs Tempering R.sub.m R.sub.p0.2 wt % ° C. Base, MPa Stipulation 0.22-0.26 3.4-6.8 0.065-0.194 1100-1250 (0.85-0.95) R.sub.m Invention Average of three sets and rounded value of data 1 0.229 3.76 0.12  980 500 610 1232 1135 2 0.240 5.30 0.094 980 500 605 1245 1146 3 0.253 6.79 0.192 980 500 635 1150 1041 4 0.238 3.46 0.093 980 500 620 1211 1108 Contrast 1 0.293 5.35 0.185 910 320 615 1145 1064 2 0.235 3.68 0.139 910 320 620 1080  995 3 0.253 7.30 0.190 910 550 600 1205 1110 4 0.258 4.25 0.196 980 320 610 1170 1123 Δ = difference between - 850 mv and stable SSRT CT CVN, Rm-CVN, corrosion corrosion Applied potential, mV Example A Z −20° C. −20° C. potential potential −850 −1200 −950 −1050 K.sub.EAC E/K.sub.EAC Weld, Z.sub.E/, or γG Base, % Base, J MPa-J mV (SCE) Z.sub.0 K.sub.IEAC E/K.sub.IEAC 0 No Stipulation 12 50 60 1100-44 Reference ≥6 Invention Average of three sets and rounded value of data 1 15 65 107  1234-66 −650 ~200 65/66 = 12/65 = 8.5 ~1 0.17 2 15 63 90 1251-58 64/63 = 10/63 = 82.6/97.2 = 8.0 ~1 0.16 0.85 Conform to KIC 3 17 67 157  −618 ~232 141.0/166.3 = 8.5 0.85 1135-98 145.5/166.3 = 0.88 Weld line 4 16 66 104  1189-90 66/65 = 12/66 = 7.5 ~1 0.18 Contrast 1 14 57 55 1135-38 48.5/57 = 7.8/56 = 6.0 0.85 0.14 2 17 69 61 1060-47 — 5.0 3 16 66 62 1195-44 −520 ~330 78.5/104.6 = 6.5 0.75 4 15 65 55 1189-20 4.0

TABLE-US-00004 TABLE 4 Results of CT test with applied potential in seawater with hydrogen precharged for 48 hours B, W, a.sub.0, K, Potential, R.sub.m, Plane Example mm mm mm MPm.sup.0.5 mV (SCE) ~MPa strain K.sub.EAC E/K.sub.EAC 0 Invention 2-1 25.13 49.86 32.442 97.2 Not applied 1250 satisfied Invention 2-2 25.14 49.84 32.476 82.6 −1050 1250 satisfied K.sub.IEACE/K.sub.IEAC0 = 82.6/97.2 = 0.85 Contrast 3-1 25.08 49.72 32.070 104.6 Not applied 1200 Not satisfied Contrast 3-2 25.08 49.70 32.138 78.5 −1050 1200 Not satisfied  78.5/104.6 = 0.75 Invention 3-1 25.14 49.88 32.108 166.3 Not applied 1150 Not satisfied Invention 3-2 25.14 49.96 32.542 141.0  −950 1150 Not satisfied 141.0/166.3 = 0.85 Invention 3-3, 25.11 50.08 32.202 145.5  −950 1135 Not satisfied 145.5/166.3 = 0.88 Weld line
Loading test: Zwick 50 kN testing machine, made by Zwick Co., Germany; prefabricated fatigue crack: MTS 810 (100 kN) electro-hydraulic servo testing machine system, made by MTS. Co., America; corrosion test device: seawater corrosion test container, equipped with slow tension and compact tension fixture; potentiostat: CHI660D electrochemical workstation. Shanghai Chenhua Instrument Co., China; pH value of artificial seawater is 8.2-7.0; 25° C. See FIG. 5-8 for specimens and tests.

[0073] The EAC test conditions are in accordance with DNVGL-CP-0237: EAC test is additionally requited for the grade R6 chain. The SSRT and K.sub.IEAC (CT) test is included to evaluate EAC resistance. The SSRT test is carried out with no potential, potential of −850 mV, −1200 mV (SCE) and axial cylindrical smooth specimen in dry atmosphere and artificial seawater. The CT test is carried out with potential of −950, −1050 mV (SCE) and in artificial seawater. Z.sub.E/Z.sub.0 and K.sub.QEAC.sub.0/K.sub.QEAC.sub.E indicate the degradation degree of EAC resistance.

[0074] The results show that there is no significant difference between SSRT data of steel in a dry atmosphere and those in an artificial seawater environment, and all of them fluctuate within the error range. SSRT of the atmospheric environment is omitted in both The carrying out example and the contrast examples.

[0075] Z.sub.0 and Z.sub.E refer to the results of the reduction of area of SSRT without and with potential respectively. In artificial seawater, the potential is not added or added to −950 mV and −1200 mV (SCE). Strain rate ≤10.sup.−5/s.

[0076] K.sub.QEAC.sub.0 and K.sub.QEAC.sub.E refer to the results of CT test without and with potential respectively. The CT specimen was precharged hydrogen for 48 hours. Tensile speed ≤6=10.sup.−9 m/s.

[0077] K.sub.QEAC.sub.0/K.sub.QEAC.sub.E refers to the degradation degree of EAC resistance in the earning out example and the contrast examples. When the K.sub.QEAC specimen meet the plane strain condition, the K.sub.IC data is obtained, and here K.sub.IEAC.sub.0 and K.sub.IEAC.sub.E refer to the results of the CT test without and with potential respectively.

[0078] According to users' requirement of forward lead EAC evaluation (Do it in advance at the steel mill), chain making and simulated quenching-tempering are carried out first, then EAC test is carried out by sampling. K.sub.QEAC of the weld zone of the chain is also tested for the comparison of the performance of the weld zone and the base.

[0079] Carrying out example (invention example) 1-4 all conform to the range of the composition limitation of the present invention. On the premise of controlling the minimum content of Ti (controlling is inevitable in the industrial scale), the limited TiN and AlN combined with a small content of N are first precipitated in the cooling process of continuous casting bloom, according to the solubility product from small to large, which ensures the subsequent precipitation of NbCN and VCN. When the heat temperature is more than 1230° C. the continuous casting bloom is forged and rolled. AlN, NbCN, VCN, M3C and M2C was first all dissolved in austenite, and then precipitated during cooling. Among them, TiN, NbCN, and AlN do not dissolve when the chain is quenched at 980° C. which hinders the growth of austenite grain. NbCN, which is still insoluble at 1150° C. is used as the main precipitate to hinder the growth of austenite grain. M3C, M2C, and VCN are fully solid solution with a high-temperature quenching at 980° C., and then precipitated again during a high-temperature tempering. The quenching-tempering steel is strengthened by fine and dense VCN, so as to make up for the loss of strengthening effect caused by reducing the total content of alloy in the present invention. There are excellent mechanical properties such as strength, plasticity and toughness and others, and it is outstanding that the low-temperature impact value of the base and the weld are higher than the standard requirements. And mechanical properties value is abundant. The processing properties are well too, which can be seen from that Bs is about 500° C. which is about 180° C. higher than contrast example's Ms 320° C., the phase transformation temperature is higher, and the crack sensitivity is lower.

[0080] In the carrying out example 1.2 and 4, the SSRT specimen was slowly stretched with applied potential −850 mV (SCE) in artificial seawater at a strain rate of ≤10.sup.−5/s, compared with the specimen without potential, the result shows that Z.sub.E/Z.sub.0=1, that is, the plasticity did not decrease. The contrast example 1, Z.sub.E/Z.sub.0=0.85. But when −1200 mV (SCE) is applied, the slow tensile specimen is seriously embrittled as Z.sub.E/Z.sub.0≤0.18 in both the earning out example 1, 2, 4 and the contrast example 1.

[0081] In the carrying out example 2, with −1050 mV (SCE) applied. K.sub.IEAC.sub.E/K.sub.IEAC.sub.0, which means that the EAC resistance did not significantly decrease. K.sub.IEAC.sub.E and K.sub.IEAC.sub.0 meet the plane strain condition and KIC criterion. This is the first K.sub.IEAC data of grade R6 steel obtained in the world.

[0082] In the carrying out example 3, with −950 mv (SCE) applied, the value of K.sub.QEAC.sub.0/K.sub.QEAC.sub.E of the chain base and the weld are 0.85 and 0.88 respectively. And the EAC resistance of the weld is higher than that of the chain base. The K.sub.QEAC very high.

[0083] In the contrast example 3, with −1050 mV (SCE) applied. K.sub.QEAC.sub.0/K.sub.QEAC.sub.E=0.75, which means that the EAC resistance decreases.

[0084] As a reference, the potential measured after immersion in seawater for 80 hours is used as the corrosion potential under laboratory conditions. The difference between corrosion potential and applied potential is overprotection potential.

[0085] Among them, the overprotection potential to −850 mV(SCE) in In the carrying out example 1 and 3 is about 200 and 232 mV (SCE) respectively, which are within the allowable range. However, the overprotection potential to −1200 mV (SCE) is about 550 and 580 mv (SCE) respectively, which is hard to bear.

[0086] The carrying out example 1, 2 is compared with the contrast example 4, the strength is increased by 62-75 MPa by adopting similar quenching-tempering treatment which shows that the strengthening effect of VCN is better than that of VC.

[0087] In the contrast example 1. Ms is low, it is sensitive to cooling crack, and C+N=0.293, which is exceeded the scope of the invention, and the impact value is unqualified. Coarse NbCN particles of 100 μm class were found. There are only VC precipitates, no VCN precipitates.

[0088] In the contrast example 2, Ms is low, and it is sensitive to cooling crack; N has a low content, N is exhausted by first precipitated NbCN, and is not enough to form AlN. The impact value is 61J, which barely conform with the standard, but the tensile strength is as low as 1080 MPa, which is unqualified. There are only VC, no VCN.

[0089] In the contrast example 0.3, the total content of alloy exceeds the scope of the invention. The difference between −850 mV and its corrosion potential of −520 mV, that is, the overprotection potential is about 0.330 mV (SCE). SSRT test show a tendency of embrittlement. The content of Nb is as high as 0.07, and NbCN is precipitated before AlN.

[0090] In the contrast example 4. Ms is low, it is sensitive to cooling crack; with the increase of Al and Ti, the total content of micro-alloy elements exceeds the scope of the invention. Due to the consumption of N, N was exhausted when NbCN was precipitated. There are only VC, no VCN. The yield ratio is 0.96 and greater than regulation 0.95. The strengthening and toughening effects are not obvious when the quenching temperature is at 980° C. Impact toughness is unqualified.

[0091] In a word, there is no VCN precipitation in the contrast examples 1.2 and 4, only VC is precipitated in tempering, and the precipitation strengthening effect of V is not ideal. And austenite grains are coarsened or begin to coarsen at 910° C. in all contrast examples: compared with all contrast example wherein there are fine austenite grains at the chain temperature of 980° C. and the tempering temperature is allowed to increase (up to 635° C. when in the contrast example 0.3), the performance and process parameters of the contrast examples are entirety lower than those of the carrying out example of the present application.

[0092] Besides the above carrying out example, the present invention further includes other carrying out examples, and any technical solution formed by equivalent transformation or equivalent substitution shall fall within the protection scope of claims of the present invention.