Steel for mooring chain of ocean platform and manufacturing method of steel

Abstract

The invention discloses steel for an ocean platform mooring chain, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.55 to 0.35 percent of C, 0.1 to 0.5 percent of Si, 1.2 to 1.5 percent of Mn, 0.6 to 1.0 percent of Cr, 0.81 to 1.0 percent of Mo, 4.2 to 5.5 percent of Ni, 0.02 to 0.10 percent of Nb, 0.02 to 0.12 percent of V, 0.001 to 0.003 percent of Ti, 0.01 to 0.05 percent of Al and less than or equal to 0.0120 percent of N. Correspondingly, the invention further discloses a manufacturing method of the steel for the mooring chain of the ocean platform. The steel for the mooring chain of the ocean platform has good strength, low-temperature toughness and plasticity.

Description

Technical Field

[0001] The present invention relates to a steel material and a manufacturing method thereof, and in particular to a high-strength steel and a manufacturing method thereof. Background Art

[0002] With the development of deep-sea marine resources, the size of offshore platforms has increased, and their structures and functions have become more complex, which has led to an increasing demand for offshore platform mooring chains. Offshore platform mooring chains are used in harsh environments and need to withstand high tensile loads, seawater impact during wave movement, seawater erosion, and microbial corrosion.

[0003] The steel used for offshore platform mooring chains currently mainly includes R3, R3S, R4, R4S, R5, R6, etc., and the corresponding tensile strengths are 690MPa, 770MPa, 860MPa, 960MPa, 1000MPa and 1100MPa, respectively. In deep sea areas with a depth of more than 1000m, the fixation of offshore platforms requires a very high load. If lower-grade mooring chain products are used, the chain diameter needs to be enlarged, which will make the chain too heavy and even cause the chain to break due to excessive deadweight load. Therefore, large offshore platforms require mooring chain steel with higher strength levels and higher impact toughness to achieve weight reduction of mooring chains under high loads.

[0004] Existing technologies usually produce high-strength steel by selecting appropriate chemical compositions and adopting controlled rolling and controlled cooling. However, the rolling and cooling processes are difficult to control and will also affect the overall uniformity of the steel's mechanical properties.

[0005] In addition, the prior art also uses a quenching and tempering process to produce high-strength steels, by optimizing the content of alloying elements and carbon elements, improving the hardenability of steel, and allowing the steel to form a martensitic structure during the cooling process. Martensite has a square lattice structure, in which carbon is dissolved in the body-centered cubic lattice of iron atoms, so that the bcc lattice is extended along the c-axis direction. This type of structure has high strength and hardness. However, high-strength steels based on martensite have poor low-temperature impact energy due to their high dislocation density. In addition, there are high-density dislocations and subgrain boundaries inside martensitic high-strength steels, and they will quickly break and fail when tiny defects such as microcracks appear during the stretching process, resulting in lower elongation.

[0006] For example, a Chinese patent document with publication number CN103667953A, publication date March 26, 2014, and titled “A low environmental crack sensitivity ultra-high strength and toughness mooring chain steel and a manufacturing method thereof” discloses that the steel composition is, by mass percentage, 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, and the remainder is Fe and unavoidable impurities. The steel has a tensile strength (Rm) ≥ 1110 MPa, a yield strength ratio (YR) of 0.88-0.92, an elongation (A) ≥ 12%, a cross-sectional shrinkage (Z) ≥ 50%, and an impact energy (Akv) at -20°C ≥ 50 J. The patent uses the above composition and two quenching processes to obtain high-strength and tough mooring chain steel.

[0007] For example, a Chinese patent document with publication number CN112011727A, publication date December 1, 2020, and titled "A Ultra-High Strength Low-Toughness Steel, Ultra-High Strength Low-Toughness Rod and Manufacturing Method Thereof" discloses that the chemical elements of the steel are, by mass percentage: 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 balance is Fe and unavoidable impurities. The mechanical properties of the steel are as follows: yield strength ≥950MPa, tensile strength ≥1150MPa, Charpy impact energy Akv ≥75J below -20°C, elongation ≥15%, and section shrinkage ≥55%. Summary of the invention

[0008] One of the purposes of the present invention is to provide a steel for offshore platform mooring chains, which is an ultra-high strength steel obtained by rationally designing the chemical composition and combining it with an optimized process, the tensile strength of which can reach 1250MPa and has good low temperature toughness.

[0009] In order to achieve the above object, the present invention provides a steel for offshore platform mooring chain, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0010] C: 0.255~0.35%;

[0011] Si: 0.1-0.5%;

[0012] Mn: 1.2-1.5%;

[0013] Cr: 0.6~1.0%;

[0014] Mo: 0.81~1.0%;

[0015] Ni: 4.2-5.5%;

[0016] Nb: 0.02~0.10%;

[0017] V: 0.02~0.12%;

[0018] Ti: 0.001~0.003%;

[0019] Al: 0.01~0.05%;

[0020] N≤0.0120%.

[0021] Furthermore, in the steel for offshore platform mooring chain of the present invention, the mass percentage of each chemical element is:

[0022] C: 0.255~0.35%;

[0023] Si: 0.1-0.5%;

[0024] Mn: 1.2-1.5%;

[0025] Cr: 0.6~1.0%;

[0026] Mo: 0.81~1.0%;

[0027] Ni: 4.2-5.5%;

[0028] Nb: 0.02~0.10%;

[0029] V: 0.02~0.12%;

[0030] Ti: 0.001~0.003%;

[0031] Al: 0.01~0.05%;

[0032] N≤0.0120%;

[0033] The balance is Fe and inevitable impurities.

[0034] In the offshore platform mooring chain steel of the present invention, the design principles of each chemical element are specifically described as follows:

[0035] C: In the steel for offshore platform mooring chains described in the present invention, the C element can improve the hardenability of the steel, so that the steel forms a low-temperature phase transformation structure with higher hardness during the quenching and cooling process, thereby improving the strength of the steel. An increase in the C element content will increase the proportion of hard phases such as martensite and lower bainite in the steel, and will also increase the number of carbide precipitation phases in the matrix, forming a dispersion strengthening effect to improve the hardness of the steel. If the C element content is too low, it will lead to low content of low-temperature phase transformation structures such as martensite and lower bainite, and it will not be possible to obtain a higher tensile strength. However, the C element content cannot be too high, otherwise it will lead to a decrease in toughness. Therefore, in the steel for offshore platform mooring chains described in the present invention, the C element content is controlled between 0.255 and 0.350%.

[0036] Si: In the steel for offshore platform mooring chains described in the present invention, Si replaces Fe atoms in the steel by substitution, forming a solid solution strengthening effect, hindering dislocation movement, and effectively improving the strength of the steel. Si is an element that inhibits the formation of ferrite and can reduce the diffusion ability of C in ferrite. Therefore, an appropriate amount of Si can avoid the formation of coarse carbides that precipitate at defects during tempering. However, too high a Si content will reduce the low-temperature impact toughness of the steel. Therefore, in the steel for offshore platform mooring chains described in the present invention, the Si content is controlled between 0.10 and 0.50%.

[0037] Mn: In the steel for offshore platform mooring chains described in the present invention, the Mn element will inhibit diffusion-type phase transformation during the quenching process of the steel, improve the hardenability of the steel, and form a low-temperature phase transformation structure. This type of structure has high strength, and the Mn element has a certain deoxidation effect. However, too high a Mn element content will lead to the formation of more residual austenite, reduce the yield strength of the steel, and the manganese-containing steel has a greater sensitivity to overheating. In addition, too high a Mn element content will cause the austenite grains to grow easily during quenching heating, and the Mn element will promote the segregation of harmful elements at the grain boundaries, increasing the temper brittleness tendency of the steel. Therefore, in the steel for offshore platform mooring chains described in the present invention, the Mn element content is controlled between 1.20 and 1.50%. The Mn element within this range is beneficial to improving the strength and hardenability of the steel, while avoiding the formation of too much residual austenite.

[0038] Cr: In the steel for offshore platform mooring chains described in the present invention, the addition of Cr element to the steel will reduce the driving force of γ-α phase transformation, inhibit the diffusion phase transformation of the steel, improve the hardenability of the steel, form a hardened martensitic structure, and obtain a steel with higher strength. At the same time, during the heating process, if the carbide of the Cr element is not completely dissolved, it will play a role in inhibiting the growth of austenite grains. If the Cr element content is too high, coarse carbides will be formed, which will deteriorate the low-temperature impact performance. Therefore, in the steel for offshore platform mooring chains described in the present invention, the Cr element content is controlled between 0.6 and 1.0% to ensure the strength and low-temperature impact performance of the steel.

[0039] Mo: In the steel for offshore platform mooring chains of the present invention, the Mo element can delay the transformation of proeutectoid ferrite, promote the formation of acicular ferrite and bainite, and improve the strength and toughness of low alloy steel. The Mo element can also increase the solid solubility of microalloying elements (Nb, V, Ti) in austenite, delay the precipitation of microalloy carbonitrides, and allow more microalloying elements to be retained until precipitation during tempering, thereby producing a greater precipitation strengthening effect. In addition, the Mo element can be dissolved into the lattice of microalloy carbonitrides precipitated in ferrite to form (M, Mo) (C, N) (M is a microalloying element), which can not only increase the volume fraction of the precipitated phase, but also significantly refine the size of the microalloy precipitates, thereby enhancing the precipitation strengthening effect. The microalloy carbonitride containing the Mo element has good thermal stability and is not easy to coarsen at high temperatures. However, the Mo element is a precious alloying element, and the addition of a higher Mo element will lead to an increase in cost. Therefore, in the steel for offshore platform mooring chains of the present invention, the Mo element content is controlled between 0.81 and 1.0% to obtain a high strength and toughness match.

[0040] Ni: In the steel for offshore platform mooring chains described in the present invention, the Ni element exists in the steel in the form of solid solution. In the component system of the present invention, the Ni element exists in the FCC phase of Fe-Ni-Mn, which can reduce the stacking fault energy and reduce the resistance to dislocation movement. In addition, the distribution of the Ni element 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 the steel. On the other hand, the Ni element is an austenite-forming element, and an excessively high Ni content will lead to an excessively high content of residual austenite in the steel, reducing the strength of the steel. Therefore, in the steel for offshore platform mooring chains described in the present invention, the Ni content is controlled between 4.2 and 5.5% to ensure the low-temperature impact toughness and strength of the steel.

[0041] Nb: In the steel for offshore platform mooring chain described in the present invention, Nb element can be added to the steel to inhibit the recrystallization of the steel. Nb element exists in the steel as a replacement solute atom. Nb atoms are larger than Fe atoms and are easily segregated at the dislocation line, which has a strong dragging effect on the movement of dislocations. At the same time, Nb element can form interstitial intermediate phases such as NbC and NbN in the steel. In the process of recrystallization, it hinders the pinning of dislocations and the migration of subgrain boundaries, can effectively refine the grains, and prevent the grain coarsening of the weld during the welding process from causing the impact performance to decrease. However, when the Nb element content is high, coarse NbC particles will be formed under high temperature tempering conditions, which will deteriorate the low-temperature impact work of the steel. Therefore, in order to cooperate with other alloy elements to ensure that the steel has good mechanical properties, the Nb element content in the steel for offshore platform mooring chain described in the present invention is controlled between 0.02 and 0.10%.

[0042] V: In the offshore platform mooring chain steel described in the present invention, V and C form VC. Small VC will have a certain hindering effect on dislocation. At the same time, VC has a high dissolution temperature, which can effectively prevent grain boundary movement, refine grains, and improve the strength of steel. However, under the condition of high temperature tempering, if the content of C and V are both high, coarse VC particles will be formed, reducing the impact resistance of steel. Therefore, in the offshore platform mooring chain steel described in the present invention, the content of V is controlled between 0.02 and 0.12% to ensure the mechanical properties of the steel.

[0043] Ti: In the steel for offshore platform mooring chains described in the present invention, the Ti element exists in the steel in the form of solid solution, and the carbonitride formed 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 element content is too high, coarse TiN will precipitate, resulting in reduced impact and fatigue properties of the steel. And during the tempering process, if the Ti element content is too high, the fluctuation amplitude of the low-temperature impact energy will increase. Therefore, in the steel for offshore platform mooring chains described in the present invention, the Ti element content is controlled between 0.001 and 0.003%.

[0044] Al: In the steel for offshore platform mooring chains described in the present invention, the Al element forms fine AlN precipitation during steelmaking, which inhibits the growth of austenite grains during the subsequent cooling process, thereby achieving the purpose of refining austenite grains and improving the toughness of steel at low temperatures. In addition, the Al element is a good deoxidizer and can effectively reduce the oxygen content in steel. However, too high an Al element content will lead to the formation of larger Al element oxides, making the steel unsuitable for ultrasonic flaw detection, and coarse aluminum oxide hard inclusions will deteriorate the fatigue properties of the steel. Therefore, in the steel for offshore platform mooring chains described in the present invention, the Al element content is controlled between 0.01 and 0.05% to improve the toughness of the steel.

[0045] N: In the offshore platform mooring chain steel of the present invention, the N element forms AlN or TiN in the steel, which plays a role in refining austenite grains. However, an increase in the N element content will lead to an increase in its enrichment at defects and the formation of coarse nitride precipitation particles, which affects the low-temperature impact energy of the steel. Therefore, in the offshore platform mooring chain steel of the present invention, the N element content is controlled to not exceed 0.012%.

[0046] The steel for the offshore platform mooring chain of the present invention adopts appropriate amounts of C, Ni and Mo elements, combined with the optimization of Mn and Cr elements, to form a tempered martensite microstructure.

[0047] Furthermore, the steel for offshore platform mooring chain of the present invention further contains at least one of the following chemical elements:

[0048] 0<Cu≤0.20%;

[0049] 0<Ca≤0.005%.

[0050] In some embodiments of the present invention, further adding at least one of Cu and Ca can further improve the performance of the steel.

[0051] When Cu is added to steel, fine nano-scale ε-Cu will be precipitated during the tempering process, which improves the strength of the steel. At the same time, adding a certain amount of Cu is beneficial to improving the corrosion resistance of the steel. However, due to the low melting point of Cu, if the Cu content is too high, copper will be enriched in the grain boundary during the heating austenitization process, resulting in the weakening of the grain boundary and cracking. Therefore, in the present invention, 0<Cu≤0.20% is controlled.

[0052] An appropriate amount of Ca element is added to steel to form CaS, which improves the size and morphology of inclusions and improves the low-temperature impact toughness of steel. However, too high a Ca content will cause the size of DS inclusions in the steel to exceed the standard, which is very unfavorable to the fatigue life of the steel. Therefore, the Ca content in the present invention can be controlled within 0<Ca≤0.005%.

[0053] Furthermore, in the steel for offshore platform mooring chains of the present invention, the mass percentage of each element satisfies at least one of the following formulas:

[0054] 0.3≤(1 / 13Cr+Nb / 8+1 / 16Mo+Ti / 4+V / 4) / C≤0.7;

[0055] 18≤(Mn+Ni) / C≤24.

[0056] Furthermore, among the inevitable impurities in the steel for offshore platform mooring chains of the present invention, the content of each impurity element satisfies at least one of the following conditions:

[0057] P≤0.015%;

[0058] S≤0.003%;

[0059] H≤0.0002%;

[0060] O≤0.003%.

[0061] In the steel for offshore platform mooring chain of the present invention, P, S, H and O are all impurities in the steel. Under the condition that the technical conditions permit, the lower the content of P, S, H and O in the steel, the better.

[0062] P: P in steel segregates at grain boundaries, which will reduce the binding energy of the grain boundaries, deteriorate the low-temperature impact performance of the steel, and aggravate the cold brittleness of the steel. P segregated at grain boundaries will cause the steel to fracture along the grain when subjected to impact loads, forming a larger cleavage plane, reducing the energy absorbed by the steel when subjected to impact. Based on this, in some embodiments of the present invention, the P content can be controlled to not exceed 0.015% to ensure the low-temperature impact toughness of the ultra-high strength steel.

[0063] S: The solubility of S in delta ferrite and austenite is very low. During the solidification of molten steel, S will be segregated to form more sulfide inclusions, which will harm the ultrasonic flaw detection performance and low-temperature impact performance of steel. In addition, S will form low-melting point FeS with Fe, showing hot brittleness. The present invention controls the S content to avoid the damage of coarse sulfides to the impact performance. Therefore, in some embodiments of the present invention, the S content can be controlled to not exceed 0.003% to ensure that the steel has good low-temperature impact performance.

[0064] H: H is affected by the hydrostatic pressure field of edge dislocations in the steel and will gather at the defects to form hydrogen embrittlement. The density of dislocations and subgrain boundaries in steels with a tensile strength of 1100MPa or above 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 delayed fracture of the steel. If rods 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 delayed cracking of high-strength mooring chains during use, endangering the safety of the offshore platform. Therefore, in some embodiments of the present invention, the H content in the steel can be controlled to not exceed 0.0002%.

[0065] O: O will form inclusions such as Al2O3 and TiO with Al in the steel, reducing the toughness of the steel. In order to ensure the uniformity of the steel structure and the low-temperature impact energy, in some embodiments of the present invention, the O content can be controlled to not exceed 0.0030%.

[0066] Furthermore, in the steel for offshore platform mooring chains of the present invention, the microstructure includes tempered martensite and precipitated carbides, wherein the volume phase ratio of the tempered martensite is greater than 90%.

[0067] Furthermore, the steel for offshore platform mooring chains of the present invention has a microstructure of tempered bainite, thereby forming a tempered martensite + tempered bainite composite structure.

[0068] Furthermore, in the steel for offshore platform mooring chains of the present invention, more than 90% of the tempered martensite laths have a width not exceeding 1 μm, and an aspect ratio of >5.

[0069] Furthermore, in the steel for offshore platform mooring chains of the present invention, the proportion of carbides with diameters below 80 nm exceeds 80% (the result is the average proportion of carbides with diameters less than 60 nm in 20 SEM fields of view).

[0070] Furthermore, the yield strength of the steel for the offshore platform mooring chain described in the present invention is ≥1100MPa, the tensile strength Rm is ≥1250MPa, the yield strength ratio is 0.85-0.95, the elongation A is ≥12%, the cross-sectional shrinkage is ≥50%, and the Charpy impact energy Akv at -20°C is ≥70J.

[0071] Accordingly, another object of the present invention is to provide a method for manufacturing steel for offshore platform mooring chains, which has a simple process flow and is easy to control. At the same time, with a reasonable chemical composition ratio, an ultra-high strength steel with a tensile strength of 1250MPa can be obtained. The steel adopts a tempering process after quenching to form a matrix structure of tempered martensite or tempered martensite + tempered bainite, and refined carbides are precipitated on the matrix, which eliminates the internal stress of the steel and has good structural uniformity.

[0072] In order to achieve the above object, the present invention also proposes a method for manufacturing steel for offshore platform mooring chains as described above, which comprises the steps of:

[0073] (1) Smelting and casting;

[0074] (2) Heating, forging or rolling, wherein the deformation per pass is controlled to be 20 to 40%;

[0075] (3) Quenching: The steel is heated to an austenitizing temperature of 850-1020°C for 60-240 min, followed by water quenching;

[0076] (4) Tempering: The tempering temperature is 570-640°C, and the tempering time is 90-300 minutes.

[0077] In the manufacturing method described in the present invention, the deformation amount of a single pass is controlled to be 20-40% because controlling the deformation amount of a single pass within this range can ensure the smooth progress of the dynamic recrystallization process, so that the matrix has fine grains and improves strength and toughness.

[0078] In addition, the present invention adopts a relatively high tempering temperature, and controls the tempering temperature to be 570-640°C. This is because: during the quenching process, steel is prone to form lower bainite and martensite structures with a large defect density, with a large strain storage energy inside, and the internal stress distribution is uneven. The present invention uses a high-temperature tempering process of 570-640°C to form fine carbides of Nb, V, C, and N. At the same time, Cr and Mo also form fine carbides and precipitate during the high-temperature tempering process, thereby improving 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, improving the phenomenon of low elongation after low-temperature tempering. Therefore, the manufacturing method of steel for offshore platform mooring chains described in the present invention can make the steel have good strength, toughness and plasticity.

[0079] In the manufacturing method of the present invention, during quenching, the rolled steel billet is heated to an austenitizing temperature of 850 to 1020°C, and then water quenched. 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 excessive coarsening of austenite grains, achieving the purpose of grain refinement after quenching, and improving the strength and toughness of the steel; during the quenching cooling process, the alloy elements dissolved in the austenite improve the hardenability of the steel, making the final martensite finer, and such a structure has ultra-high strength and good toughness.

[0080] Furthermore, in some embodiments of the manufacturing method described in the present invention, in step (2), the ingot can be heated once and rolled or forged once to directly roll or forge the ingot to the size of the final product, wherein the heating temperature is 1050-1250°C, and the final rolling or final forging temperature is controlled to be 850-1020°C.

[0081] Furthermore, in some other embodiments of the manufacturing method described in the present invention, in step (2), the ingot can be first heated and rolled or forged for the first time, and the ingot can be rolled into an intermediate billet, and then the intermediate billet can be heated and rolled or forged for the second time to obtain the final finished product size, wherein the first heating temperature is 1050-1250°C, and the first final rolling or final forging temperature is 850-1020°C; the second heating temperature is 1070-1150°C, and the second final rolling or final forging temperature is 870-980°C.

[0082] Furthermore, the manufacturing method of the present invention further includes air cooling after step (2) and before step (3).

[0083] Furthermore, in the manufacturing method of the present invention, air cooling or water cooling is performed after step (4).

[0084] The steel for offshore platform mooring chain and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0085] The offshore platform mooring chain steel of the present invention can form a tempered martensite or tempered martensite + tempered bainite microstructure and precipitate fine carbides by limiting the appropriate range of alloy elements, thereby exhibiting ultra-high strength and toughness mechanical properties, and can be used to produce R7 offshore platform mooring chain bars.

[0086] In some embodiments, the yield strength is ≥1100 MPa, the tensile strength Rm is ≥1250 MPa, the yield strength ratio is 0.85-0.95, the elongation A is ≥12%, the cross-sectional shrinkage is ≥50%, and the Charpy impact energy Akv at -20°C is ≥70J.

[0087] The manufacturing method of the steel for the offshore platform mooring chain of the present invention can achieve high strength and toughness matching by only using one quenching process, thus saving the quenching process and reducing the production cost and carbon emission. BRIEF DESCRIPTION OF THE DRAWINGS

[0088] Figure 1 A photograph of the steel for offshore platform mooring chain in Example 4 at 500 times magnification under an optical microscope is shown.

[0089] Figure 2 A photograph of the steel for offshore platform mooring chain of Example 4 under a scanning electron microscope at 2000 times magnification is shown. DETAILED DESCRIPTION

[0090] The steel for offshore platform mooring chain and the manufacturing method thereof according to the present invention will be further explained and illustrated below in conjunction with the accompanying drawings and specific embodiments of the specification. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0091] The steel for offshore platform mooring chains of Examples 1-8 and Comparative Examples 1-5 were prepared by the following steps:

[0092] 1) Smelting: The molten steel is smelted in an electric furnace or a converter according to the composition ratios shown in Table 1-1 and Table 1-2.

[0093] 2) Casting: The molten steel obtained from smelting is cast into billets or ingots by mold casting or continuous casting.

[0094] 3) Heating; forging or rolling: The cast billet or ingot is heated, and then forged or rolled to obtain a billet.

[0095] Examples 3, 5-6 and Comparative Examples 2-3 and Comparative Example 5 only perform heating and rolling once, and the ingot is directly rolled into the final size of the finished product, wherein the heating temperature is 1050-1250°C, and the final rolling or final forging temperature is 850-1020°C.

[0096] In Examples 1-2, Example 4, and Comparative Examples 1 and 4, the ingots are first heated and rolled for the first time, and the ingots are rolled to a specified intermediate billet size, and then the intermediate billet is heated and rolled for a second time, and the intermediate billet is rolled to the size of the final product, wherein the first heating temperature is 1050-1250°C, and the first final rolling or final forging temperature is 850-1020°C; the second heating temperature is 1070-1150°C, and the second final rolling or final forging temperature is 870-980°C.

[0097] 4) Air-cool the rolled steel billet.

[0098] 5) Quenching: The rolled steel billet is heated to an austenitizing temperature of 850 to 1020°C for 60 to 240 minutes, and water quenching is performed after austenitizing.

[0099] 6) Tempering: The tempering temperature is 570-640°C, and the tempering time is 90-300 minutes.

[0100] 7) Air cooling or water cooling: Examples 1-4 use air cooling, and Examples 5-6 use water cooling.

[0101] Table 1-1 and Table 1-2 list the mass percentage ratios of the chemical elements of the offshore platform mooring chain steels of Examples 1-8 and the comparative steels of Comparative Examples 1-5.

[0102] Table 1-1 (wt.%, the balance is Fe and other unavoidable impurities except P, S, H and O)

[0103]

[0104]

[0105] Table 1-2 (wt.%, the balance is Fe and other unavoidable impurities except P, S, H and O)

[0106]

[0107] Table 2 lists the specific process parameters of the offshore platform mooring chain steels of Examples 1-8 and the comparative steels of Comparative Examples 1-5 in the above process steps.

[0108] Table 2

[0109]

[0110]

[0111] The present invention also samples the steel for offshore platform mooring chains of Examples 1-8 and Comparative Examples 1-5, observes their microstructures using a scanning electron microscope, and lists the observation results in Table 3.

[0112] Table 3.

[0113]

[0114]

[0115] also, Figure 1 The optical microscope photo of the steel used for offshore platform mooring chain in Example 4 is shown. Figure 1 It can be seen that the microstructure of the steel is tempered martensite.

[0116] Figure 2 The following is a photo of the steel used for offshore platform mooring chain in Example 4 at 2000 times magnification under a scanning electron microscope. Figure 2 It can be seen that the carbides are distributed in a fine dispersed state.

[0117] In order to verify the implementation effects of the embodiments of the present invention, samples of the offshore platform mooring chain steels of embodiments 1-8 and comparative examples 1-5 were taken for mechanical property testing, and the test results are listed in Table 4.

[0118] in:

[0119] Yield strength and tensile strength Rm are determined as follows: GB / T228.1-2010

[0120] Charpy impact energy Akv (-20℃) is determined as follows: GB / T 229

[0121] The elongation A is determined as follows: GB / T228.1-2010

[0122] The cross-sectional shrinkage Z is determined as follows: GB / T228.1-2010

[0123] Table 4.

[0124]

[0125]

[0126] Note: The longitudinal impact energy Akv at -20℃ in Table 4 records the values ​​of three sets of parallel tests.

[0127] It can be seen from Table 4 that the yield strength of the steels of Examples 1 to 8 of the present invention is greater than 1100 MPa, the tensile strength is greater than 1250 MPa, the yield strength ratio is 0.889 to 0.912, the Charpy impact energy Akv at -20°C is greater than 70 J, the elongation is greater than or equal to 14.5%, and the cross-sectional shrinkage is greater than or equal to 58%. This fully demonstrates that the steel for mooring chains of the present invention has good strength, low-temperature toughness and plasticity.

[0128] On the other hand, in Comparative Example 1, the C content is insufficient, and the strength of the steel is significantly reduced, which cannot meet the requirement of a tensile strength of 1250 MPa.

[0129] The Mn content of Comparative Example 2 exceeds the scope of the present invention, which will cause the material's yield ratio to exceed 0.95, which is not conducive to service safety.

[0130] The Ni content of Comparative Example 3 is insufficient, resulting in a sharp deterioration in the low-temperature impact toughness of the material, and the number of oversized carbides will increase, deteriorating the impact toughness.

[0131] The Cr content of Comparative Example 4 is higher than the range of the present invention, which will lead to coarsening of carbide size, deterioration of impact energy, and reduction of the yield strength ratio of the material.

[0132] In Comparative Example 5, due to the small deformation amount in the rolling pass, the dynamic recrystallization process of the material is limited, and the resulting fine grain strengthening effect is limited, resulting in a decrease in toughness.

[0133] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0134] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. A steel for offshore platform mooring chain, containing Fe and unavoidable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.255~0.35%, Si: 0.1~0.5%, Mn: 1.2~1.5%, Cr: 0.6~1.0%, Mo: 0.81~1.0%, Ni: 4.2~5.5%, Nb: 0.02~0.10%, V: 0.02~0.12%, Ti: 0.001~0.003%, Al: 0.01~0.05%, N≤0.0120%.

2. The steel for offshore platform mooring chain according to claim 1, characterized in that: The mass percentage of each chemical element is: C: 0.255~0.35%, Si: 0.1~0.5%, Mn: 1.2~1.5%, Cr: 0.6~1.0%, Mo: 0.81~1.0%, Ni: 4.2~5.5%, Nb: 0.02~0.10%, V: 0.02~0.12%, Ti: 0.001~0.003%, Al: 0.01~0.05%, N≤0.0120%; the balance is Fe and other inevitable impurities.

3. The steel for offshore platform mooring chain according to claim 1 or 2, characterized in that: It also contains at least one of 0<Cu≤0.20% and 0<Ca≤0.005%.

4. The steel for offshore platform mooring chain according to claim 1 or 2, characterized in that: The mass percentage of each element satisfies at least one of the following formulas: 0.3≤(1 / 13Cr+Nb / 8+1 / 16Mo+Ti / 4+V / 4) / C≤0.7; 18≤(Mn+Ni) / C≤24.

5. The steel for offshore platform mooring chain according to claim 1 or 2, characterized in that: Among other inevitable impurities, the content of each impurity element satisfies at least one of the following conditions: P≤0.015%, S≤0.003%, H≤0.0002%, O≤0.003%.

6. The steel for offshore platform mooring chain according to claim 1 or 2, characterized in that: The microstructure thereof includes tempered martensite and precipitated carbides, wherein the volume phase ratio of tempered martensite is greater than 90%.

7. The steel for offshore platform mooring chain according to claim 6, characterized in that: Its microstructure also includes tempered bainite.

8. The steel for offshore platform mooring chain according to claim 6, characterized in that: More than 90% of the tempered martensite has a lath width of no more than 1 μm and an aspect ratio of >5.

9. The steel for offshore platform mooring chain according to claim 6, characterized in that: The proportion of carbides with diameters below 80nm exceeds 80%.

10. The steel for offshore platform mooring chain according to claim 1 or 2, characterized in that: Its yield strength is ≥1100MPa, tensile strength Rm is ≥1250MPa, yield strength ratio is 0.85-0.95, elongation A is ≥12%, cross-sectional shrinkage is ≥50%, and Charpy impact energy Akv at -20℃ is ≥70J.

11. The method for manufacturing steel for offshore platform mooring chains according to any one of claims 1 to 10, characterized in that: It includes the steps of: (1) Smelting and casting; (2) Heating, forging or rolling, wherein the deformation per pass is controlled to be 20 to 40%; (3) Quenching: The steel is heated to an austenitizing temperature of 850-1020°C for 60-240 min, followed by water quenching; (4) Tempering: The tempering temperature is 570-640°C, and the tempering time is 90-300 minutes.

12. The manufacturing method according to claim 11, characterized in that: In step (2), the ingot is heated once and rolled or forged once to directly roll or forge the ingot to the size of the final product, wherein the heating temperature is 1050-1250°C and the final rolling or final forging temperature is controlled to be 850-1020°C.

13. The manufacturing method according to claim 11, characterized in that: In step (2), the ingot is first heated and rolled or forged for the first time, and the ingot is rolled into an intermediate billet, and then the intermediate billet is heated and rolled or forged for the second time to obtain the final finished product size, wherein the first heating temperature is 1050-1250°C, and the first final rolling or final forging temperature is 850-1020°C; the second heating temperature is 1070-1150°C, and the second final rolling or final forging temperature is 870-980°C.

14. The manufacturing method according to claim 11, characterized in that: Air cooling is also included after step (2) and before step (3).

15. The manufacturing method according to claim 11, characterized in that: After step (4), air cooling or water cooling is performed.

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