Steel for ultra-large-specification offshore platform mooring chain with tensile strength of 1200 MPa and manufacturing method thereof

Abstract

The invention provides steel with tensile strength of 1200 MPa for an oversized ocean platform mooring chain and a manufacturing method of the steel. The steel for the mooring chain comprises more than 90% of Fe and inevitable impurities and further comprises the following chemical elements in percentage by mass: 0.250%-0.35% of C, 0.10%-0.50% of Si, 1.20%-1.50% of Mn, 0.80%-1.20% of Cr, 0.60%-1.00% of Mo, 2.60%-3.60% of Ni, less than or equal to 0.20% of Cu, 0.04%-0.15% of Nb, 0.02%-0.12% of V, 0.001%-0.003% of Ti, 0.0010%-0.0050% of B, less than or equal to 0.005% of Ca, 0.01%-0.05% of Al and less than or equal to 0.0120% of N. The high-strength and high-toughness steel has good strength, low-temperature toughness and plasticity, and can be used for manufacturing ultra-large R7-grade high-performance ocean platform mooring chains and the like. The invention further provides a manufacturing method of the steel for the mooring chain.

Description

Technical Field

[0001] The present invention relates to the field of steel for marine platform mooring chains, and in particular to steel for super-large marine platform mooring chains with a tensile strength of 1200 MPa and a manufacturing method thereof. Background Art

[0002] With the development of deep-sea marine resources, offshore platforms are becoming larger, more complex in structure, and more functional, placing increasing demands on offshore platform mooring chains. Offshore platform mooring chains operate in harsh environments, needing to withstand high tensile loads, wave impact, seawater erosion, and microbial corrosion. Currently, offshore platform mooring chain steels primarily include R3, R3S, R4, R4S, R5, and R6. Their corresponding tensile strengths are 690 MPa, 770 MPa, 860 MPa, 960 MPa, 1000 MPa, and 1100 MPa, respectively. In deep-sea areas exceeding 1000 meters, securing offshore platforms requires high loads. Using lower-grade mooring chain products requires increasing the chain diameter, which increases the chain's deadweight and can even lead to chain breakage due to the excessive deadweight. Therefore, large offshore platforms require mooring chain steels with higher strength grades and improved impact toughness to reduce the weight of the mooring chains under these high loads. As the scale of marine platforms used in the mid- and deep-sea areas becomes larger and larger, the requirements for their carrying capacity are also getting higher and higher. Some small and medium-sized mooring chains can no longer meet their use requirements. Therefore, it is necessary to develop extra-large mooring chain bars with large size, high tensile strength, good low-temperature impact toughness and elongation. At present, no steel mill can provide steel that meets the requirements.

[0003] A major challenge in large-scale mooring chains is achieving martensite under existing heat treatment conditions. In the commonly used quenching and tempering process, the quenching time for a single large-scale mooring chain typically exceeds 5 minutes. The first section to be immersed in water cools rapidly, allowing for martensite formation. However, the subsequent section, however, cools more slowly, preventing complete martensite transformation. Consequently, the strength at the link locations does not meet the required level. Summary of the Invention

[0004] In view of the above technical problems, a first aspect of the present invention provides a steel for a mooring chain. In addition to containing more than 90% of Fe and unavoidable impurities, the steel further contains the following chemical elements in terms of mass percentage:

[0005] C: 0.250~0.350%, Si: 0.10~0.50%, Mn: 1.20~1.50%, Cr: 0.80~1.20%, Mo: 0.60~1.00%, Ni: 2.60~3.60%, Cu≤0.20%, Nb: 0.04~0.15%, V: 0.02~0.12%, Ti: 0.001~0.003%, B: 0.0010~0.0050%, Ca≤0.005%, Al: 0.01~0.05%, N≤0.0120%.

[0006] Preferably, the mooring chain steel contains the following chemical elements in mass percentage:

[0007] C: 0.250~0.350%, Si: 0.10~0.50%, Mn: 1.20~1.50%, Cr: 0.80~1.20%, Mo: 0.60~1.00%, Ni: 2.60~3.60%, Cu≤0.20%, Nb: 0.04~0.15%, V: 0.02~0.12%, Ti: 0.001~0.003%, B: 0.0010~0.0050%, Ca≤0.005%, Al: 0.01~0.05%, N≤0.0120%, and the balance is Fe and unavoidable impurities.

[0008] Preferably, the impurity elements in the mooring chain steel meet the following requirements in terms of mass percentage: P≤0.015%, S≤0.003%, H≤0.0002%, and O≤0.0030%.

[0009] Preferably, the microstructure of the steel for the mooring chain contains tempered martensite and tempered bainite; more preferably, the microstructure of the steel for the mooring chain is tempered martensite and tempered bainite; even more preferably, the microstructure of the steel for the mooring chain contains 90 volume % or more of tempered martensite and 10 volume % or less of tempered bainite.

[0010] Preferably, the yield strength of the mooring chain steel is ≥1000 MPa, for example, 1000-1100 MPa, the tensile strength Rm is ≥1200 MPa, for example, 1200-1300 MPa, the Charpy impact energy Akv at -20°C is ≥65 J, preferably Akv ≥80 J, for example, 85-110 J, the elongation A is ≥12%, preferably A ≥14%, for example, 14-18%, and the reduction of area is ≥50%, for example, 50-65%.

[0011] Preferably, the strength-plasticity-toughness product Rm*A*Akv of the mooring chain steel is above 1500 GPa%J, such as 1500-2000 GPa%J, preferably above 1800 GPa%J, such as 1800-2000 GPa%J.

[0012] A second aspect of the present invention provides a method for manufacturing the above-mentioned steel for offshore platform mooring chains, comprising the following steps performed in sequence:

[0013] 1) smelting and casting molten steel to obtain ingots;

[0014] 2) heating and forging or rolling the ingot to obtain a rolled or forged steel billet;

[0015] 3) quenching the rolled or forged steel billet; and

[0016] 4) Tempering;

[0017] 5) Deep cryogenic treatment.

[0018] Preferably, in step 2), the ingot is heated to 1050-1250°C. Within this temperature range, the carbonitrides of Nb and V and the carbides of Cr and Mo partially or completely dissolve in the austenite. During the subsequent rolling / forging and cooling process, Nb and V form fine carbonitrides that pin the austenite grain boundaries and refine the rolled steel structure. Cr and Mo dissolved in the austenite improve the hardenability of the steel and enhance the hardenability of the martensite during quenching.

[0019] Preferably, in step 2), the ingot is heated and rolled or forged once, and the ingot is rolled or forged to the size of the final product, wherein the heating temperature is 1050-1250°C, and the final rolling or forging temperature is 900-1020°C; or, in step 2), the ingot is first heated and rolled or forged for the first time, and the ingot is rolled or forged to the specified intermediate billet size, and then the intermediate billet is heated and rolled or forged for the second time, and the intermediate billet is rolled or forged to the size of the final product, wherein the temperature of the first heating is 1050-1250°C, and the final rolling or forging temperature of the first rolling or forging is 900-1020°C; the temperature of the second heating is 1070-1150°C, and the final rolling or forging temperature of the second rolling or forging is 910-980°C.

[0020] The steel of the present invention is austenitized by heating at 1050°C to 1250°C. During heating, Nb and V carbonitrides and Cr and Mo carbides partially or completely dissolve in the austenite. During subsequent rolling / forging and cooling, Nb and V form fine carbonitrides that pin austenite grain boundaries and refine the as-rolled steel structure. Furthermore, Cr and Mo dissolved in the austenite improve the steel's hardenability and, during quenching, enhance the hardenability of the martensite.

[0021] Under the condition that the final rolling or final forging temperature is greater than or equal to 900℃, the steel undergoes recrystallization and strain-induced precipitation, forming a refined bainite and martensite multiphase matrix structure with fine carbonitride precipitation.

[0022] Preferably, after rolling or forging and before quenching, the rolled or forged steel billet is air-cooled or slowly cooled.

[0023] Preferably, the cooling rate of air cooling is 60° C. / h or less, and the cooling rate of slow cooling is 15° C. / h or less.

[0024] Preferably, the rolled steel billet is a rolled bar / round steel, and the final size D of the bar / round steel is above 150 mm, preferably 150 to 240 mm. The size of the bar / round steel herein refers to the diameter of the bar / round steel.

[0025] Preferably, during quenching, the rolled steel billet is heated to an austenitizing temperature of 880 to 1020°C, and the quenching heating time is 210 to 360 minutes. After austenitizing, water quenching is performed, and the quenching cooling rate is above 2°C / s, preferably below 10°C / s, and the final cooling temperature is below 100°C. By heating the steel billet to this temperature range, the carbonitrides of Nb, V, Ti, Cr and Mo are completely or partially dissolved, thereby avoiding the austenite grains from being too coarse, achieving the purpose of grain refinement after quenching, and improving the strength and toughness of the steel; during the quenching cooling process, the alloying elements dissolved in the austenite improve the hardenability of the steel, making the final martensite finer, and this type of structure has ultra-high strength and good toughness.

[0026] By controlling the quenching cooling rate to 2-10℃ / s, the entire martensitic structure can be obtained.

[0027] Preferably, the tempering temperature is 570-630°C, and the tempering time is 240-400 minutes. During the quenching process, the steel forms lower bainite and martensite structures with a large defect density, a large strain storage energy inside, and an uneven internal stress distribution. During the high-temperature tempering process, Nb and V will form fine carbides with C and N. At the same time, Cr and Mo will also form fine carbides and precipitate during the high-temperature tempering process, which improves the strength and toughness matching of the steel; at the same time, due to the annihilation of high-density dislocations and the movement of small-angle grain boundaries, the microstructure of the steel is more uniform, which improves the phenomenon of low elongation after low-temperature tempering. Within the tempering temperature range described in the present invention, it can be ensured that the steel has good strength, toughness and plasticity. The tempering process of the present invention can effectively reduce the internal stress of the steel, which is beneficial to the processing and use of bars / round steel, such as the production of high-performance R7 offshore platform mooring chains.

[0028] This invention, through rationally designed chemical composition and optimized processing, has developed a low-temperature, ultra-high-strength, high-toughness steel with a tensile strength of 1200 MPa. After quenching, the rolled or forged bars / round steel are tempered to form a tempered martensite and tempered bainite matrix structure. Fine carbide precipitation within the matrix eliminates internal stress in the steel and provides excellent structural uniformity.

[0029] The steel material of this invention features a rational composition and process design, a wide process window, and can be commercially produced in large quantities on bar or plate production lines. The steel produced using this invention exhibits a yield strength of 1000 MPa or greater, a tensile strength of 1200 MPa or greater, a Charpy impact energy (Akv) of 65 J (-20°C), an elongation of 12%, and a reduction of area of ​​50%. This ultra-high-strength steel exhibits excellent strength, low-temperature toughness, and ductility, and can be used to produce ultra-large R7 offshore platform mooring chains. DETAILED DESCRIPTION

[0030] 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.

[0031] In this paper, yield strength, tensile strength Rm, elongation A and reduction of area Z are determined according to GB / T 228.1-2010;

[0032] Charpy impact energy Akv (-20℃) is determined in accordance with GB / T 229-2020.

[0033] Table 1 shows the compositions of the steel in the prior art and the steel for mooring chains in the present invention. It can be seen that the chemical composition of the steel for mooring chains in the present invention is quite different from that of the steel in the prior art.

[0034] For example, Chinese patent application CN103667953A (published on March 26, 2014) discloses a low environmental crack sensitivity ultra-high strength and toughness mooring chain steel and a manufacturing method thereof. The steel composition, in mass percentage, is as follows: C 0.12-0.24, Mn 0.10-0.55, Si 0.15-0.35, Cr 0.60-3.50, Mo 0.35-0.75, N≤0.006, Ni 0.40-4.50, Cu≤0.50, S≤0.005, P 0.005-0.025, O≤0.0015, H≤0.00015, with the remainder being Fe and unavoidable impurities. The steel has a tensile strength (Rm) ≥ 1110 MPa, a yield ratio (YR) of 0.88-0.92, an elongation (A) ≥ 12%, a reduction of area (Z) ≥ 50%, and an impact energy (Akv) at -20°C ≥ 50 J. This high-performance marine mooring chain steel also contains one or more of the microalloying elements V, Al, Nb, Ti, Ca, and RE, with the following contents expressed in mass percentages: V 0.05-0.25, Al 0.01-0.06, Nb 0.005-0.05, Ti 0.002-0.10, Ca 0.005-0.004, RE 0.010-0.025, and Sn+Sb+As ≤ 0.035. This patent utilizes the above composition and a double quenching process to produce high-strength and tough mooring chain steel.

[0035] On the one hand, the composition of the mooring chain steel of the present invention differs from that of CN103667953A in the content of C, Mn, Mo, Ti, and rare earth elements (RE). Specifically, the present invention utilizes the design of C, Ni, Mo, and rare earth elements (RE), combined with the optimization of elements such as Mn and Cr, to form a tempered bainite and tempered martensite duplex microstructure, which differs from the microstructure formed by the steel in CN103667953A (upper bainite + lower bainite + martensite, upper bainite + lower bainite, or upper bainite).

[0036] On the other hand, the organizational design ideas for achieving high strength and toughness in the technical solutions of the present invention and CN103667953A are completely different, so the component systems and process systems used to form such organizations are completely different. The difference between the preparation method of the present invention patent and that in CN103667953A is that the quenching treatment mentioned in CN103667953A is a two-time quenching treatment, while the steel of the present invention can achieve high strength and toughness matching by using a single quenching process, saving the quenching process, reducing production costs and carbon emissions, and is an environmentally friendly steel. In addition, the tensile strength of the steel in the present invention reaches more than 1200MPa, and the mechanical properties are significantly better.

[0037] Chinese patent application CN112011727A discloses an ultra-high strength and low temperature toughness steel, an ultra-high strength and low temperature toughness bar and a manufacturing method thereof. The chemical elements of the steel are, by mass percentage, as follows: C: 0.150-0.250%, Si: 0.10-0.50%, Mn: 0.60-1.50%, Cr: 0.30-1.20%, Mo: 0.20-0.80%, Ni: 2.00-4.00%, Nb: 0-0.10%, B: 0.0010-0.0050%, V: 0-0.12%, Ti: 0.003-0.06%, Al: 0.01-0.08%, and the remainder is Fe and unavoidable impurities. The mechanical properties of the steel are as follows: yield strength ≥950MPa, tensile strength ≥1150MPa, Charpy impact energy Akv below -20℃ ≥75J, elongation ≥15%, and section shrinkage ≥55%.

[0038] The difference between the composition of the steel in the present invention and that in CN112011727A lies in the completely different contents of C, Mo, and Ti. The present invention does not contain B, and at the same time optimizes the composition range of alloying elements such as Mn, Mo, Nb, and V, and limits the content of the H element. The use of the alloying element range defined by the present invention enables the steel to form tempered martensite and tempered bainite microstructures, exhibiting ultra-high strength and toughness mechanical properties. In addition, the tensile strength of the steel for mooring chains of the present invention is greater than or equal to 1200MPa, and the yield strength is greater than or equal to 1000MPa, and the strength grade is significantly higher than that of the steel in CN112011727A. The technical solution of the present invention is completely different from the technical route of CN112011727A in terms of steel organization design, composition, and process design.

[0039] In the steel for offshore platform mooring chains of the present invention, the design principles of the chemical elements are specifically described as follows:

[0040] C: Adding C to steel improves its hardenability and causes the steel to form a high-hardness low-temperature phase transformation structure during the quenching and cooling process, thereby increasing its strength. Increasing the C content increases the proportion of hard phases, such as martensite and lower bainite, in the steel. It also increases the amount of carbide precipitation in the matrix, creating a dispersion strengthening effect that increases the hardness of the steel. However, too low a C content results in low levels of low-temperature phase transformation structures, such as martensite and lower bainite, and a high tensile strength cannot be achieved. However, the C content cannot be too high, otherwise it will lead to a decrease in toughness. Therefore, in the present invention, the C content is 0.250 to 0.350 wt.%.

[0041] Si: Si replaces Fe atoms in steel by substitution, forming a solid solution strengthening effect, hindering dislocation movement, and effectively improving the strength of steel. Si is an element that inhibits the formation of ferrite and can reduce the diffusion ability of C in ferrite. Therefore, during tempering, an appropriate amount of Si can prevent the formation of coarse carbides that precipitate at defects. However, too high a Si content will reduce the low-temperature impact toughness of the steel. Therefore, in the present invention, the Si content is: 0.10~0.50wt.%.

[0042] Mn: During the quenching process of steel, Mn inhibits diffusion-type phase transformation, improves the hardenability of the steel, and forms a low-temperature phase transformation structure. This structure has higher strength, and Mn also has a certain deoxidizing effect. However, too high a Mn content will lead to the formation of more retained austenite, which reduces the yield strength of the steel. At the same time, manganese-containing steel is more sensitive to overheating. In addition, too high a Mn content will cause the austenite grains to grow more easily during quenching heating. At the same time, Mn will promote the segregation of harmful elements at grain boundaries, increasing the tendency of temper brittleness of the steel. Therefore, the Mn content in the present invention is 1.20-1.50wt.%. Mn within this range is beneficial to improving the strength and hardenability of the steel while avoiding the formation of excessive retained austenite.

[0043] Phosphorus (P) segregates at grain boundaries in steel, reducing its binding energy, deteriorating the steel's low-temperature impact resistance, and exacerbating its cold brittleness. P segregating at grain boundaries can cause intergranular fracture when subjected to impact loads, forming larger cleavage planes and reducing the energy absorbed by the steel during impact. In the present invention, the P content does not exceed 0.015 wt.% to ensure the low-temperature impact toughness of the ultra-high-strength steel.

[0044] S: S has very low solubility in delta ferrite and austenite. During solidification, S segregates, forming numerous sulfide inclusions that impair the steel's ultrasonic flaw detection and low-temperature impact properties. Furthermore, S reacts with Fe to form low-melting-point FeS, which exhibits hot brittleness. The present invention controls the S content to prevent coarse sulfides from impairing impact properties. Therefore, the S content in the present invention is limited to no more than 0.003 wt.% to ensure good low-temperature impact properties.

[0045] Cr: Adding Cr to steel reduces the driving force of the γ-α phase transformation, inhibits the steel's diffusion-type phase transformation, improves the steel's hardenability, forms a hardened martensitic structure, and produces steel with higher strength. Furthermore, if the Cr carbides are not completely dissolved during heating, they inhibit the growth of austenite grains. Excessive Cr content forms coarse carbides, deteriorating low-temperature impact properties. Therefore, the Cr content in the present invention is 0.80 to 1.20 wt.% to ensure the steel's strength and low-temperature impact properties.

[0046] Mo: Mo delays the proeutectoid ferrite transformation, promotes the formation of acicular ferrite and bainite, and improves the strength and toughness of low-alloy steel. Mo also increases the solid solubility of microalloying elements (Nb, V, Ti) in austenite, delaying the precipitation of microalloying carbonitrides, allowing more microalloying elements to be retained for precipitation during tempering, thereby producing a greater precipitation strengthening effect. Furthermore, Mo can dissolve into the lattice of microalloying carbonitrides precipitated in ferrite, forming (M, Mo)(C, N) (M being a microalloying element). This not only increases the volume fraction of the precipitated phase but also significantly refines the size of the microalloying precipitates, thereby enhancing the precipitation strengthening effect. Mo-containing microalloying carbonitrides have good thermal stability and are not susceptible to coarsening at high temperatures. However, Mo is a precious alloying element, and adding higher amounts of Mo can increase costs. Therefore, the Mo content in the present invention is 0.60-1.00 wt.% to achieve a high strength and toughness match.

[0047] Nb: Nb is added to steel to inhibit the recrystallization of the steel. Nb exists in the steel as a replacement solute atom. Nb atoms are larger than Fe atoms and tend to be segregated at dislocation lines, which has a strong dragging effect on the movement of dislocations. At the same time, Nb can form interstitial phases such as NbC and NbN in steel. During the recrystallization process, it hinders the pinning of dislocations and the migration of subgrain boundaries, effectively refining the grains and preventing the coarsening of the weld grains during welding, which causes a decrease in impact performance. However, a high Nb content will form coarse NbC particles under high-temperature tempering conditions, which will deteriorate the low-temperature impact energy of the steel. Therefore, in order to cooperate with other alloying elements to ensure that the steel has good mechanical properties, the Nb content in the present invention is 0.04 to 0.15 wt.%.

[0048] Ni: Ni exists in the form of solid solution in steel. In the composition system of the present invention, Ni exists in the form of Fe-Ni-Mn FCC phase, which can reduce the stacking fault energy and reduce the resistance to dislocation movement. In addition, the distribution of Ni on the θ / α interface reduces the carbon flux to 0, thereby hindering the growth of cementite, effectively preventing the abnormal growth of carbides, improving the toughness of the steel matrix, and improving the low-temperature impact performance of steel. On the other hand, Ni is an austenite-forming element, and too high a Ni content will lead to an excessively high content of retained austenite in the steel, reducing the strength of the steel. Therefore, the Ni content in the present invention is 2.60 to 3.60 wt.% to ensure the low-temperature impact toughness and strength of the steel.

[0049] Cu: When added to steel, Cu forms fine, nanoscale ε-Cu precipitates during tempering, increasing the steel's strength. A certain amount of Cu also helps improve the steel's corrosion resistance. However, due to its low melting point, excessive Cu content can lead to copper accumulation at grain boundaries during austenitization, weakening them and potentially causing cracking. Therefore, the Cu content in this invention should not exceed 0.20 wt.%.

[0050] V: In steel, V forms V with C. Fine V has a certain inhibitory effect on dislocations. Its high dissolution temperature effectively prevents grain boundary movement, refines grains, and improves steel strength. However, under high-temperature tempering conditions, if both C and V content are high, coarse V particles will form, reducing the steel's impact resistance. Therefore, the V content in the present invention is 0.02-0.12 wt.% to ensure the steel's mechanical properties.

[0051] Al: Al forms fine AlN precipitates during steelmaking, which inhibit austenite grain growth during the subsequent cooling process, thereby refining the austenite grains and improving the steel's toughness at low temperatures. Furthermore, Al is a good deoxidizer, effectively reducing the oxygen content in steel. However, excessive Al content can lead to the formation of large Al oxides, making the steel unsuitable for ultrasonic testing. Furthermore, coarse aluminum oxide hard inclusions can deteriorate the steel's fatigue properties. Therefore, the Al content in the present invention is 0.01 to 0.05 wt.% to improve the steel's toughness.

[0052] Ti: Ti exists in the form of solid solution in steel. The carbonitrides it forms can pin the austenite grain boundaries, prevent the austenite grains from being too coarse, achieve the purpose of grain refinement after quenching, and improve the strength and toughness of the steel. However, if the Ti content is too high, coarse TiN precipitation will form, resulting in reduced impact and fatigue properties of the steel. Moreover, during the tempering process, if the Ti content is too high, the fluctuation amplitude of the low-temperature impact energy will increase. Therefore, the Ti content in the present invention is controlled at 0.001 to 0.003 wt.%.

[0053] B: Because this invention involves ultra-large mooring chain bars / round steel, the heat treatment method places stringent demands on the hardenability of these bars / round steels. B effectively improves the hardenability of alloy steel, enabling high-strength martensitic structures to be achieved even at lower cooling rates. However, excessive B content can increase the brittleness of the steel. Therefore, the present invention adds 0.0010-0.0050 wt.% of B.

[0054] Ca: An appropriate amount of Ca added to steel can form CaS, improve the size and morphology of inclusions, and enhance the low-temperature impact toughness of the steel. However, excessive Ca content can lead to excessive DS inclusions in the steel, affecting the fatigue life of the steel. Therefore, in the present invention, the Ca content is controlled below 0.005wt.%.

[0055] H: H is affected by the hydrostatic pressure field of edge dislocations in the steel and will gather at defects, forming hydrogen embrittlement. The density of dislocations, subgrain boundaries, etc. in steel with a tensile strength level of more than 1100MPa is high. If the H content is too high, more H atoms will be enriched at the defects after the steel is quenched and tempered. The aggregation of H atoms will form H molecules, causing the steel to undergo delayed fracture. If rods / round steel with a high H content are used to produce offshore platform mooring chains, the corrosion of the mooring chains by seawater and the penetration of H will cause the high-strength mooring chains to undergo delayed cracking during use, endangering the safety of the offshore platform. Therefore, the H content in the steel of the present invention does not exceed 0.0002wt.%.

[0056] N: N forms AlN or TiN in steel, which refines austenite grains. However, an increase in N content leads to increased N enrichment at defects and the formation of coarse nitride precipitation particles, which affects the low-temperature impact energy of the steel. Therefore, the N content in the present invention does not exceed 0.0120 wt.%.

[0057] O: O will form Al2O3, TiO and other inclusions with Al in steel, reducing the toughness of the steel. To ensure the uniformity of the steel structure and low-temperature impact energy, the O content shall not exceed 0.0030%.

[0058] In the present invention, P, S, H, and O are all impurities in the steel. The lower their content in the steel, the purer the steel and the better its performance. The chemical composition design of the present invention fully utilizes the effects of various alloying elements on phase transformation and microstructure, forming a multiphase microstructure dominated by tempered martensite and tempered bainite. Simultaneously, the contents of P, S, N, O, and H are controlled to ensure the steel's strength, low-temperature impact toughness, and elongation. This facilitates the production of high-strength steel with a tensile strength of 1200 MPa, combining ultra-high strength and toughness with high ductility.

[0059] The technical solution of the present invention is further described in detail below with reference to the examples. It should be understood that the following examples are only used to describe the specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention.

[0060] Examples 1-6 and Comparative Examples 1-8

[0061] The mooring chain steels of Examples 1-6 and Comparative Examples 1-8 of the present invention were prepared by the following steps:

[0062] 1) Smelting: smelting the molten steel in an electric furnace or converter according to the formula shown in Table 2;

[0063] 2) Casting: Molten steel obtained from smelting is cast into billets or ingots by die casting or continuous casting;

[0064] 3) Heating and forging or rolling: The cast billet or ingot is heated and forged or rolled to obtain a rolled or forged billet.

[0065] Specifically, the ingots in Examples 3, 5-6 and Comparative Examples 3-8 are rolled or forged to the final size of the finished product, wherein the heating temperature is 1050-1250°C, and the final rolling or final forging temperature is 900-1020°C; the ingots in Examples 1-2 and 4 and Comparative Example 1-2 are first heated and rolled or forged for the first time, and the ingots are rolled / forged to the specified intermediate billet size, and then the intermediate billet is heated and rolled or forged for the second time, and the intermediate billet is rolled or forged to the size of the final product, wherein the temperature of the first heating is 1050-1250°C, and the final rolling or final forging temperature of the first rolling or forging is 900-1020°C; the temperature of the second heating is 1070-1150°C, and the final rolling or final forging temperature of the second rolling or forging is 910-980°C.

[0066] Preferably, the rolled steel billet is a rolled bar / round steel, and the final size D of the bar / round steel is above 150 mm, preferably 150 to 240 mm.

[0067] 4) The rolled steel billet is air-cooled or slowly cooled: Examples 1-3 and Comparative Examples 1-4 use air cooling at a cooling rate of 60°C / h or less, and Examples 4-6 and Comparative Examples 5-8 use slow cooling at a cooling rate of 15°C / h or less.

[0068] 5) Quenching: During quenching, the rolled steel billet is heated to an austenitizing temperature of 880-1020°C for 210-360 minutes. After austenitizing, it is water quenched at a quenching cooling rate of 2-10°C / s and a final cooling temperature of below 100°C.

[0069] 6) Tempering: The tempering temperature is 570-630°C, and the tempering time is 240-400 minutes.

[0070] 7) Cryogenic treatment time T = (0.12-0.15) * D (diameter of the bar or round steel) * X, where X = 0.54C (5.1Mn-1.12) (1 + 0.7Si) (1 + 0.36Ni) (1 + 2.16Cr) (1 + 3Mo) (1 + 0.36Cu) (1 + 1.73V). When the unit of the bar or round steel diameter is mm, the unit of T is min.

[0071] When calculating X, substitute the value before the percentage sign for the corresponding element content. For example, if the carbon content in the steel is 0.250%, substitute 0.250 into the above formula for calculation. X is dimensionless. Table 4 shows the cryogenic treatment times for Examples 1-6 and Comparative Examples 1-8. The actual cryogenic treatment time T is between Tmin(0.12*D*X) and Tmax(0.15*D*X).

[0072] The steels of Comparative Examples 1-4 were prepared in the same manner as Example 1, with the only difference being the chemical composition of the steels.

[0073] The steels of Comparative Examples 5-8 were prepared in substantially the same manner as in Example 1, with the only difference being that the process parameters used in step 3), step 5) or step 6) were different.

[0074] The specific process parameters of Examples 1-6 and Comparative Examples 1-8 are shown in Tables 3 and 4 below.

[0075] The steels of Examples 1-6 and Comparative Examples 1-8 were sampled, and the microstructure and mechanical properties of each steel sample were tested. The test results are shown in Table 5. The longitudinal impact energy Akv at -20°C in Table 5 records the values ​​of three sets of parallel tests. The product of strength-ductility-toughness Rm*A*Akv was calculated by inserting the average of the three normal test results.

[0076] As can be seen from Table 5, the steels of Examples 1-6 of the present invention have a yield strength greater than or equal to 1000 MPa, a tensile strength greater than or equal to 1200 MPa, a Charpy impact energy Akv (-20°C) greater than or equal to 65 J, an elongation greater than or equal to 12%, a reduction of area greater than or equal to 50%, and a strength-plasticity-toughness product (Rm*A*Akv) greater than 1500 GPa%J. The mooring chain steel of the present invention exhibits excellent strength, low-temperature toughness, and ductility, and can be used to manufacture R7-grade high-performance offshore platform mooring chains and ultra-high-strength and toughness structural steel. Comparative Examples 1-4 exhibit significantly lower strength and low-temperature impact toughness compared to Examples 1-6 due to elemental content outside the specified range of the present invention. The process parameters of Comparative Examples 5-8 are also outside the specified range of the present invention, resulting in uncontrollable microstructure content in the steel and inability to fully balance mechanical properties, thus failing to meet the requirements for R7-grade mooring chain steel.

[0077] All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0078] Although the present invention has been described with reference to certain preferred embodiments thereof, it should be understood by those skilled in the art that the above description is provided to further illustrate the present invention in conjunction with specific embodiments thereof, and that the present invention should not be construed as being limited to these descriptions. Those skilled in the art may make various changes in form and detail, including simple deductions or substitutions, without departing from the spirit and scope of the present invention.

[0079]

[0080]

[0081]

[0082]

[0083]

Claims

1. A steel for a mooring chain, characterized in that: The mooring chain steel contains, in addition to more than 90% of Fe and unavoidable impurities, the following chemical elements in terms of mass percentage: C: 0.250~0.350%, Si: 0.10~0.50%, Mn: 1.20~1.50%, Cr: 0.80~1.20%, Mo: 0.60~1.00%, Ni: 2.60~3.60%, Cu≤0.20%, Nb: 0.04~0.15%, V: 0.02~0.12%, Ti: 0.001~0.003%, B: 0.0010~0.0050%, Ca≤0.005%, Al: 0.01~0.05%, N≤0.0120%.

2. The mooring chain steel according to claim 1, characterized in that: The mooring chain steel contains the following chemical elements in mass percentage: C: 0.250~0.350%, Si: 0.10~0.50%, Mn: 1.20~1.50%, Cr: 0.80~1.20%, Mo: 0.60~1.00%, Ni: 2.60~3.60%, Cu≤0.20%, Nb: 0.04~0.15%, V: 0.02~0.12%, Ti: 0.001~0.003%, B: 0.0010~0.0050%, Ca≤0.005%, Al: 0.01~0.05%, N≤0.0120%, and the balance is Fe and unavoidable impurities.

3. The mooring chain steel according to claim 1 or 2, characterized in that: The impurity elements satisfy the following in percentage by mass: P≤0.015%, S≤0.003%, H≤0.0002%, O≤0.0030%.

4. The mooring chain steel according to claim 1 or 2, characterized in that: The microstructure of the mooring chain steel comprises tempered martensite and tempered bainite; preferably, the microstructure of the mooring chain steel comprises tempered martensite and tempered bainite; more preferably, the microstructure of the mooring chain steel comprises more than 90 volume % of tempered martensite and less than 10 volume % of tempered bainite.

5. The mooring chain steel according to claim 1 or 2, characterized in that: The mooring chain steel has a yield strength of ≥1000 MPa, a tensile strength Rm ≥1200 MPa, a Charpy impact energy Akv at -20°C ≥65 J, preferably Akv ≥80 J, an elongation A ≥12%, preferably A ≥14%, and a cross-sectional shrinkage Z ≥50%.

6. The mooring chain steel according to claim 5, characterized in that: The product of strength-plasticity-toughness Rm*A*Akv of the mooring chain steel is above 1500 GPa%J, preferably above 1800 GPa%J.

7. A method for manufacturing the steel for a mooring chain according to any one of claims 1 to 6, characterized in that: The method comprises the following steps performed in sequence: 1) smelting and casting molten steel to obtain ingots; 2) heating and forging or rolling the ingot to obtain a rolled or forged steel billet; 3) quenching the rolled or forged steel billet; and 4) Tempering; 5) Deep cryogenic treatment.

8. The method according to claim 7, wherein In step 2), the cast slab is heated to 1050-1250°C.

9. The method according to claim 7, wherein In step 2), the ingot is heated and rolled or forged once, and the ingot is rolled or forged to the size of the final product, wherein the heating temperature is 1050-1250°C, and the final rolling or forging temperature is 900-1020°C; alternatively, in step 2), the ingot is first heated and rolled or forged for the first time, and the ingot is rolled or forged to the specified intermediate billet size, and then the intermediate billet is heated and rolled or forged for the second time, and the intermediate billet is rolled or forged to the size of the final product, wherein the temperature of the first heating is 1050-1250°C, and the final rolling or forging temperature of the first rolling or forging is 900-1020°C; the temperature of the second heating is 1070-1150°C, and the final rolling or forging temperature of the second rolling or forging is 910-980°C.

10. The method according to claim 9, wherein After rolling or forging and before quenching, the rolled or forged steel billet is air-cooled or slowly-cooled; preferably, the cooling rate of air cooling is less than 60°C / h, and the cooling rate of slow cooling is less than 15°C / h; Preferably, the steel billet after rolling or forging is a bar or round steel, and the final size D of the bar or round steel is above 150 mm, preferably D is 150-240 mm.

11. The method according to claim 7, wherein During quenching in step 3), the rolled or forged steel billet is heated to an austenitizing temperature of 880 to 1020° C. for a quenching heating time of 210 to 360 minutes. After austenitizing, water quenching is performed at a quenching cooling rate of more than 2° C. / s, and the final cooling temperature is less than 100° C.

12. The method according to claim 7, wherein In step 4), the tempering temperature is 570-630° C., and the tempering time is 240-400 minutes.

13. The method according to claim 7, wherein In step 5), the cryogenic treatment time T = (0.12-0.15) * the diameter of the bar or round steel * X, where X = 0.54C (5.1Mn-1.12) (1 + 0.7Si) (1 + 0.36Ni) (1 + 2.16Cr) (1 + 3Mo) (1 + 0.36Cu) (1 + 1.73V). When the unit of the bar or round steel diameter is mm, the unit of T is min.

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