US20210285069A1 - 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 - Google Patents

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 Download PDF

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US20210285069A1
US20210285069A1 US17/258,174 US201917258174A US2021285069A1 US 20210285069 A1 US20210285069 A1 US 20210285069A1 US 201917258174 A US201917258174 A US 201917258174A US 2021285069 A1 US2021285069 A1 US 2021285069A1
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chain
grade
mooring
toughness
strength
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Jiang Yin
Xuejun Bao
Zhenwei Liu
Yun Bai
Xiaohong XU
Ming Zhang
Haidong FAN
Yao Li
Hongli Li
De Chen
Guorong Zhu
Guoren HU
Tiehua YAO
Qiuhao GU
Yong Cui
Erxin HUANG
Xuewen Liu
LI Liangliang
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Jiangyin Xingcheng Special Steel Works Co Ltd
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Jiangyin Xingcheng Special Steel Works Co Ltd
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Assigned to JIANGYIN XINGCHENG SPECIAL STEEL WORKS CO., LTD reassignment JIANGYIN XINGCHENG SPECIAL STEEL WORKS CO., LTD ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: CUI, YONG, GU, Qiuhao, HUANG, Erxin, LI, Liangliang, LIU, XUEWEN, XU, XIAOHONG, YAO, Tiehua, BAI, YUN, BAO, XUEJUN, CHEN, DE, FAN, HAIDONG, HU, Guoren, LI, HONGLI, LI, YAO, LIU, ZHENWEI, ZHANG, MING, ZHU, GUORONG, YIN, Jiang
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Definitions

  • the present application relates to alloy steel and ferrous metallurgical products for offshore engineering, in particular a grade R6 steel and chain for offshore mooring in the steel series for offshore mooring chain and the evaluation of its offshore environmental performance degradation resistance.
  • a mooring system is acquired for positioning and mooring with floating bodies in the process of the marine exploration, deep-sea oil and gas extinction, national defense construction and so on.
  • the main component of the mooring system is offshore mooring chain by the steel.
  • DNVGL Det Norske Veritas-Gemtanischer Lloyd
  • Offshore Standards DNVGL - OS - E 302 Offshore mooring chain. Edition July 2018, the offshore mooring chain can be classified into six grades called R3, R3S, R4, R4S, R5, and R6 according to its strength grading.
  • the tensile strength of the chains at all grades after overall quenching-tempering shall be equal or greater than 690 MPa, 770 MPa, 860 MPa, 960 MPa, 1000 MPa, and 1100 MPa respectively.
  • the grade R5 is the highest grade of the mooring chain produced and used at home and abroad by the first half of the year of 2018.
  • the long mooring chain for positioning can be divided into two types which are stud and studless for drilling type and production type of the floating bodies respectively.
  • the length of a single chain can reach several kilometers. Alloy steel bars of diameter 52-230 min are used accordingly. All offshore mooring chain steels have an ultrahigh strength, as marine steel with a tensile strength above 690 MPa is ultrahigh-strength steel according to DNVGL.
  • the qualified mechanical properties and service performance of the offshore mooring chain are requisite in the offshore environment.
  • its toughness especially the toughness of the flash weld area, must be ensured for the safety and reliability for resisting wild winds and huge waves.
  • the chain is usually continuously treated in a special vertical furnace, i.e. the final performance of the chain is obtained after continuous quenching and tempering heat treatment.
  • the accessories are forged from the steel, and their final properties are obtained through intermittent heat treatment.
  • DNVGL has defined grade R6 chain as the extreme technology and performance of the offshore mooring chain since 2013, and the provisional standard was not be terminated until 2018. DNVGL officially announced a standards including the grade R6 chain, specifying the performance of the chain and the basic process requirements of the production of the chain steel in July 2018.
  • DNVGL also proposed an additional standard named Class programme - DNVGL - CP -0237 , Edition July 2018 , Offshore mooring chain and accessories which evaluates the EAC (environment assisted cracking) of grade R6 chain under CP (cathodic protection) conditions according to the standards of ASTM G129 and ASTM E1820.
  • the specific requirement is to carry out two tests: the one is SSRT with the applied potential of ⁇ 850 mV, ⁇ 1200 mV (saturated calomel electrode SCE) and strain rate of ⁇ 10 ⁇ 5 /s, the other is the K IEAC test (fracture toughness type 1 in seawater) with CT (compact tension) specimen, applied potential of ⁇ 950 mV, ⁇ 1050 mV (SCE) and the test speed of ⁇ 6 ⁇ 10 ⁇ 9 m/s in seawater. So as to evaluate the degradation resistance of grade R6 chain in the seawater environment.
  • the seawater environment is generally considered as the service forbidden zone for ultrahigh-strength steel with yield strength over 1000 MPa.
  • the main way to lighten the chain is ultrahigh intensification. There is a restraining relationship each other between strength and toughness, and the same is true of the relationship between strength and deterioration resistance of service performance in the seawater environment. Once the strength is improved, there must be a decrease of toughness, plasticity and the degradation resistance in the seawater environment performance. But according to DNVGL, when the strength is increased, the toughness is increased instead of decreasing with its EAC evaluated at the same time.
  • DNVGL proposes the above additional requirements for the evaluation of EAC of the chain by impressed cathodic protection potential of the grade R6 chain for the sake of reliability. This is a new challenge in the research and development of the ultrahigh-strength chain.
  • the invention has passed the acceptance of the expert committee organized by the Ministry of industry and information technology, as actural product, the steel and the chain in the invention meet and exceed the temporary standards of mechanical properties and seawater environmental service performance of the grade R6 chain, Vanadium are the element for refining or strengthening effect in the form of VC in the invention.
  • the invention does not cover the technology to balance corrosion potential and cathodic protection potential to prevent embrittlement by hydrogen generation of the chain caused by cathodic overprotection, coping with new standard.
  • the content of evaluating the embrittlement by hydrogen generation caused by cathodic protection is added in the latest standards of DNVGL2018 under the premise of ensuring the mechanical properties of the chain. While the reliability of the chain mainly depends on the overall performance given by the steel.
  • the toughness of base and weld of the chain are reduced below the qualified line by coarse eutectic niobiumcarbide.
  • the air cooling process is impossible to be implemented in the continuous heat treatment production line to avoid the tow-temperature phase transformation crack although this patent requires air cooling after heat treatment.
  • retained austenite will decompose during the high-temperature tempering although austenite is beneficial to toughness and environmental performance.
  • the technical problem to be solved by the present invention is to propose a new manufacturing scheme of a grade R6 offshore mooring chain as well as its steel for use in anchoring and mooring floating bodies with cathodic protection.
  • the precipitation strengthening effect and the toughness and a low corrosion rate of the chain are improved on the premise of ensuring the hardenability and reduces the corrosion potential when the total contents of alloy and microalloy are narrowed, to curb the embrittlement by hydrogen generation due to the chains are received by cathodic overprotection passively.
  • the chemical composition by wt % 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 rest is Fe and unavoidable impurity elements.
  • the content of N in the chemical composition is 0.016-0.024.
  • the chain is made of round bars corresponding to the above chain steel.
  • the present invention further limits the content of C and N and the range of ⁇ M and ⁇ MM on the basis of the related product composition applied by two the Chinese invention patents whose publication numbers are CN103667953 B and CN106636928A.
  • the main deoxidizing element of the steel in the present invention is Al, which is used for sufficient peroxidation. And the content of residual Al after the final deoxidation is controlled no more than 0.02 to reduce the consumption of N caused by Al.
  • the Nb content in the present invention is limited to 0.02-0.05, and NbCN is precipitated from the steel, and the effect of NbCN inhibiting the growth of austenite grain is stronger than that of NbC.
  • the chain temperature is increased from ⁇ 920° C. to ⁇ 980° C. during the quenching heating, and microstructure of the chain after austenitizing is transformed into BU (upper bainite) during cooling.
  • the initial phase transformation temperature of BU and martensite is Bs and Ms, respectively, and Bs( ⁇ 500° C.) is higher then Ms( ⁇ 320° C.).
  • the cooling crack sensitivity decreases with the increase of the phase transformation temperature.
  • the nitride and carbonitride are controlled to be precipitated in the sequence of TiN—AlN—NbCN—VCN, on the basis of the general technology that is smelting, bloom continuous casting, blooming with high-temperature heating, intermediate billet heating, forging or rolling, chain making, flash welding and heat treatment and so on.
  • the solubility product of [Nb] [C+N] is smaller, and the effect of inhibiting the growth of austenite grain of [Nb] [C+N] is stronger than that of NbC.
  • the existence of NbCN allows the temperature of the chain to increase from ⁇ 920° C. to ⁇ 980° C. before quenching. In addition, there is to the drag effect of solid solution of Nb on the grain boundary.
  • MCN is also expressed as VCN in engineering.
  • (2.3) Al is used as the main deoxidizing element and the subordinate element to increase the austenite coarsening temperature.
  • the consumption of N by Al can be reduced, and the residual content of N combined with Nb and V can be increased, that is to say, NbCN and VCN can be formed with the former effectively improving the austenite coarsening temperature and the latter increasing the strengthening effect.
  • the chain link is transformed into a composite bainite microstructure consisting of BU (upper bainite) with a small amount of BL (lower bainite) and M (martensite) during the cooling process after austenitizing.
  • BU upper bainite
  • BL lower bainite
  • M martensite
  • phase transformation temperature of BU with Bs ⁇ 500° C. is higher than that of the martensite with Ms of ⁇ 320° C.
  • the cooling crack sensitivity decreases with the increase of the phase transformation temperature.
  • Corrosion potential the flat specimen from 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, reference calomel electrode);
  • SSRT is carried out in the atmosphere and in artificial seawater, with no potential or with potential of ⁇ 850 mV, ⁇ 1200 mV (SCE); SSRT with axial cylindrical smooth specimen whose strain rate is ⁇ 10-5/s; Z 0 and Z E are the reductions of area measured with no potential and with potential after specimens are broken respectively; the Z 0 /Z E ratio indicates the degradation degree of EAC resistance.
  • K IEAC test of CT specimen in artificial seawater, hydrogen is precharged for 48 hours when the potential is not applied, or applied by ⁇ 950 mV and ⁇ 1050 mV (SCE). Then, stretch with a speed of ⁇ 6*10 ⁇ 9 m/s, when no potential is applied, or the potential of ⁇ 950 mV and ⁇ 1050 mV (SCE) is applied.
  • K QEAC 0 and K QEAC E are the fracture toughness of specimen without potential and with potential respectively; K QEAC 0 /K QEAC E indicates the degradation degree of EAC resistance.
  • the K IC data is obtained, and the EAC here is expressed as K IEAC E , K IEAC 0
  • K QEAC of the weld zone of the chain is also tested for comparing the performance of the weld zone with that of the base.
  • the chain of the present invention is made of round bar having the 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 with ⁇ 980° C. and tempering with 600° C. to 690° C. Quenching and tempering are cooled with water less than 50° C.
  • Round bar is made from continuous casting bloom or ingot having corresponding chemical composition, and 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 micro-alloying elements MM and the limit of C+N, the precipitation sequence of nitride and carbonitride is TiN—AlN—NbCN-MCN.
  • the nitride and carbonitride are controlled to be precipitated in the sequence of TiN—AlN—NbCN-MCN on the basis of the general technology, that is smelting-bloom continuous casting-blooming with high temperature heating-intermediate billet heating-forging or rolling-chain making-flash butt welding-heat treatment, and so on.
  • the solubility product of [Nb][C+N] is smaller, the effect of inhibiting the growth of austenite grain of NbCN is stronger than that of NbC, and the existence of NbCN (and solid solution of Nb) allows the temperature of the chain to increase from ⁇ 920° C. to ⁇ 980° C. before quenching.
  • 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%;
  • FIG. 2 CCT schematic curve of the present invention
  • 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:
  • the size of the effective grain for toughening is smaller because of the existence of substructure
  • 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;
  • FIG. 5 The sampling schematic of CT test and z-x direction specimen is adopted in the invention.
  • FIG. 6 The sire of CT specimen according to DNVGL.
  • FIG. 7 SSRT test by applied potential
  • FIG. 8 CT test by applied potential
  • 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.
  • EAC test is additionally requited for the grade R6 chain.
  • the SSRT and K 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 E /Z 0 and K QEAC 0 /K QEAC E indicate the degradation degree of EAC resistance.
  • Z 0 and Z E refer to the results of the reduction of area of SSRT without and with potential respectively.
  • the potential is not added or added to ⁇ 950 mV and ⁇ 1200 mV (SCE). Strain rate ⁇ 10 ⁇ 5 /s.
  • K QEAC 0 and K QEAC 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 ⁇ 9 m/s.
  • K QEAC 0 /K QEAC E refers to the degradation degree of EAC resistance in the earning out example and the contrast examples.
  • K IC data is obtained, and here K IEAC 0 and K IEAC E refer to the results of the CT test without and with potential respectively.
  • chain making and simulated quenching-tempering are carried out first, then EAC test is carried out by sampling.
  • K QEAC of the weld zone of the chain is also tested for the comparison of the performance of the weld zone and the base.
  • Carrying out example (invention example) 1-4 all conform to the range of the composition limitation of the present invention.
  • 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.
  • 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.
  • TiN, NbCN, and AlN do not dissolve when the chain is quenched at 980° C.
  • NbCN 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.
  • K IEAC E /K IEAC 0 which means that the EAC resistance did not significantly decrease.
  • K IEAC E and K IEAC 0 meet the plane strain condition and KIC criterion. This is the first K IEAC data of grade R6 steel obtained in the world.
  • 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.
  • 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.
  • the overprotection potential to ⁇ 1200 mV (SCE) is about 550 and 580 mv (SCE) respectively, which is hard to bear.
  • 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.
  • 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.
  • 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).
  • SCE stainless steel
  • SSRT test show a tendency of embrittlement.
  • the content of Nb is as high as 0.07, and NbCN is precipitated before AlN.
US17/258,174 2019-04-16 2019-05-16 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 Pending US20210285069A1 (en)

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CN201910306407.X 2019-04-16
PCT/CN2019/087168 WO2020211137A1 (zh) 2019-04-16 2019-05-16 一种适用于锚泊定位阴极保护浮体的r6级高强韧性海洋系泊链钢及其系泊链

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