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.

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 −5 /s; Z 0  and Z 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 e /Z 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 −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 QEAC     0   , and K QEAC     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 QEAC     0    and K QEAC     E    with applied potential of −1050 mV are met plane strain conditions, the criteria of KIC is satisfied, and the value of K IEAC     E   /K IEAC     0    is 0.85; with an applied potential of −950 mV, the value of K QEAC     0   /K QEAC     E    of the welding line is 0.88; When K QEAC     0   /K QEAC     E   ≥0.80, the reduction of EAC resistance is within the controllable range, and the values of K IC  and K 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.