Steel for forging anchor chain and mooring chain accessory and manufacturing method thereof

Abstract

The invention discloses steel for forging anchor chain and mooring chain accessories and a manufacturing method thereof, and belongs to the technical field of steel production. The steel is medium-carbon high-alloy MnCrNiMo series steel, and chemical components of the steel are accurately controlled so as to give consideration to obdurability, corrosion resistance and fatigue resistance. According to the manufacturing method, a combined process of KR-BOF-LF-RH composite smelting, composite electromagnetic stirring continuous casting, high-pressure continuous rolling and stress relief annealing is adopted to replace a traditional electroslag remelting process. According to the produced steel with the specification of phi 200-350 mm, the internal quality meets the EN 10228-3 standard 4-level ultrasonic flaw detection requirement, the mechanical property meets the CCS classification society standard, the-20 DEG C impact energy is larger than or equal to 80 J, and the hydrogen embrittlement Z1 / Z2 is larger than or equal to 0.85; compared with the prior art, the energy consumption is reduced by more than 30%, the production period is shortened by 50%, the manufacturing cost is reduced by 25-30%, mooring chain accessories for deep sea energy equipment can be supplied in batches, and remarkable economic and environmental benefits are achieved.

Description

Technical Field

[0001] This invention belongs to the field of production technology of steel for high-strength anchor chain and mooring chain accessories. Specifically, it relates to a process for preparing R4 grade 4 mooring chain forging accessories using continuous casting large round billets and high-pressure rolling technology. It is suitable for the mass production of mooring chain accessories required for deep-sea energy development (wind power, hydropower, nuclear power, etc.). Background Technology

[0002] Anchor chain mooring chain accessories are core components in marine and deep-sea energy equipment, connecting chain links to the anchor body and mooring structures. These include enlarged chain links, end chain links, shackles, and swivels. Their service environment involves long-term exposure to harsh conditions such as seawater immersion, alternating loads, and low-temperature impacts. Therefore, the steel used for these accessories must simultaneously meet multiple requirements, including high strength, high toughness, excellent weldability, corrosion resistance, wear resistance, and fatigue resistance, and must be certified by a classification society (such as CCS).

[0003] As energy development extends from shallow to deep waters, the demand for high-grade (R4) mooring chain accessories continues to grow. Currently, the production of steel for R4 mooring chain forged accessories generally uses electroslag remelted steel ingots as intermediate billets, which are then forged into finished products. This process has the following key drawbacks: High energy consumption and high cost: The electroslag remelting process consumes a lot of electrical energy and the steel ingot preparation cycle is long, resulting in low production efficiency and high overall cost; Insufficient internal quality stability: During the solidification process, electroslag steel ingots are prone to defects such as central segregation, shrinkage cavities, and porosity, which are difficult to completely eliminate in subsequent forging, affecting the fatigue resistance of accessories; Poor environmental performance: Electroslag remelting generates a lot of pollutants, which does not conform to the industrial development trend of low-carbon and environmental protection. Limited adaptability to large sizes: It is difficult to efficiently produce intermediate billets with a diameter of Φ200-350mm and above, which cannot meet the needs of deep-sea equipment for large mooring chain accessories.

[0004] Therefore, developing a low-energy, low-cost, high-quality R4 fourth-level mooring chain accessory steel manufacturing process suitable for large-scale production has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a process for producing high-strength R4 grade 4 mooring chain forging steel by continuous casting large round billets and high-pressure rolling technology. The process adopts a reasonable smelting and continuous casting process and high-pressure rolling technology to produce Φ200-350mm hot-rolled intermediate billets of mooring chain forging steel. The internal quality meets the requirements of EN 10228-3 standard level 4 ultrasonic flaw detection, and the mechanical properties meet the requirements of CCS classification society standards.

[0006] The technical solution adopted by the present invention to solve the above problems is as follows: a steel for forging anchor chain and mooring chain accessories, the chemical composition of which, by weight percentage, is: C: 0.18-0.28%, Si: 0.15-0.30%, Mn: 1.00-1.80%, P: ≤0.015%, S: ≤0.010%, Cr: 0.40-1.30%, Mo: 0.20-0.60%, Ni: 0.40-1.40%, Nb: 0.01-0.06%, Cu≤0.20%, H≤1.5ppm, with the balance being iron and unavoidable impurity elements.

[0007] The working principle of each element: C: Controlled within the range of 0.18-0.28%, balancing the strength and toughness of steel, avoiding excessive carbon content which leads to decreased toughness, and excessively low carbon content which fails to meet high strength requirements.

[0008] Mn, Cr, Ni, and Mo: synergistically improve the hardenability, toughness, and seawater corrosion resistance of steel. Among them, Mo can significantly improve the fatigue resistance of steel, making it suitable for alternating loads in deep-sea environments.

[0009] Nb: By refining austenite grains, it inhibits grain growth during rolling, further improving the strength and toughness balance of steel.

[0010] P, S, H: Strictly control at low levels. P and S easily form brittle phases, leading to cold brittleness and hot brittleness in steel. H can cause white spot defects. Low content control can avoid the above problems and ensure the safety of steel in service.

[0011] This invention also provides a method for manufacturing steel for anchor chain and mooring chain accessories, the method comprising six steps: steel smelting, continuous casting, rolling, stress-relief annealing, inspection, and forging, as detailed below: 1. Steel smelting The composite smelting process adopted is "KR pretreatment - BOF converter smelting - LF refining - RH vacuum degassing". Molten iron is first pretreated by KR to remove sulfur and silicon, ensuring that the sulfur content is ≤0.005% before entering the converter; The BOF converter mainly completes the removal of phosphorus and carbon and heating. Alloys are added during tapping to initially adjust the composition. The LF refining stage involves fine-tuning of composition, deoxidation, deep desulfurization, and removal of inclusions to ensure the purity of molten steel. The RH vacuum degassing treatment controls the vacuum degree to ≤67Pa and the degassing time to ≥20min. Finally, the P, S, and H are controlled to be ≤0.015%, ≤0.010%, and ≤1.5ppm when the steel is tapped, producing medium-carbon high-alloy MnCrNiMo molten steel with low impurities and high purity.

[0012] The composite smelting process can achieve multi-step purification: KR pretreatment efficiently removes sulfur, RH vacuum degassing deeply removes hydrogen and gaseous inclusions, and LF refining ensures uniform composition. The three work together to ensure that the quality of molten steel meets the requirements of subsequent continuous casting.

[0013] 2. Continuous casting Argon gas is used for full-process protection during casting to prevent secondary oxidation of the molten steel; Equipped with a composite electromagnetic stirrer of crystallizer M-EMS and end F-EMS, with a stirring current of 150-200A and a frequency of 2-4Hz; Control the superheat of molten steel to 20–35℃, the flow rate of secondary cooling water to 0.8–1.2 L / kg, and the casting speed to 0.4–0.6 m / min; After the continuously cast billet (Φ700mm large round billet) is taken off the production line at ≥500℃, it is immediately placed in a slow cooling pit and kept at the temperature for ≥40h. After being taken out of the pit, it is promptly put into the furnace for rolling. The low-magnification microstructure of continuously cast billets must be free from defects such as shrinkage cavities, bubbles, cracks, white spots, and surface defects.

[0014] Composite electromagnetic stirring drives the flow of molten steel through electromagnetic force, breaking the growth trend of columnar crystals, expanding the proportion of equiaxed crystal regions (≥60%), refining grains and reducing compositional segregation; low casting speed and narrow superheat control further ensure uniform and dense solidification structure, and slow cooling treatment can release internal stress in the billet and avoid cracks during cooling.

[0015] 3. Rolling The billet is heated in a three-section walking beam furnace with a heating rate of ≤60℃ / h, a heating temperature of 1220~1270℃, and a holding time of 14~16h to ensure thorough firing of the billet; The initial rolling temperature is 950–1100℃, and the final rolling temperature is ≥850℃. A 1350mm large reduction rolling mill is used for multi-stand continuous rolling, with a total reduction rate ≥70%. The reduction is controlled in three stages: primary rolling, intermediate rolling, and finishing rolling. The reduction rate is 30–40% for primary rolling, 35–45% for intermediate rolling, and 15–25% for finishing rolling. During the rolling process, high-pressure water descaling (water pressure ≥18MPa) is used to remove iron oxide scale and avoid surface defects; After being rolled into finished products (Φ200-350mm) at ≥300℃, they are placed in a slow cooling pit and removed from the pit after being cooled to below 200℃.

[0016] High-reduction rolling technology allows the rolling force to penetrate into the core of the billet, breaking down defects such as porosity and shrinkage cavities formed during solidification and increasing the density of the steel. Segmented reduction control can avoid surface cracks caused by excessive single reduction. Reasonable heating and rolling temperatures ensure that the steel is in the austenite recrystallization zone, ensuring uniform mechanical properties.

[0017] 4. Stress-relief annealing Annealing temperature 520~580℃, holding time 12~20h; After cooling in the furnace to below 250°C, remove from the furnace and air-cool to room temperature.

[0018] Stress-relief annealing can eliminate residual stress generated during rolling, preventing deformation and cracking caused by stress release during subsequent forging or service, while not affecting the strength and toughness of the steel.

[0019] 5. Inspection After the beginning and end of each rolled stock are cut off, ultrasonic testing of level 4 is performed according to EN 10228-3 standard to ensure that there are no internal defects. Low-magnification microstructure testing should be performed on batch samples (ASTM E381 standard), requiring S≤2.0, R≤2.0, and C≤2.0. Mechanical property testing shall be conducted according to GB / T2975 standard, with samples taken at 1 / 6D position. Standard specimens shall be prepared after heat treatment of the blank, and the properties shall meet the following requirements: yield strength ReH≥580MPa, tensile strength Rm≥860MPa, elongation≥12%, reduction of area≥50%, yield ratio≤0.92, V-notch impact energy KV2≥80J at -20℃, and hydrogen embrittlement Z1 / Z2≥0.85. Tensile strength, impact resistance, hydrogen embrittlement and other indicators must comply with CCS classification society standards.

[0020] 6. Forging The heating rate of the intermediate billet is ≤200℃ / h, the forging heating temperature is 1240~1280℃, and the holding time is 4~6h; Initial forging temperature ≤1250℃, final forging temperature ≥800℃, control forging reduction to ensure forging force penetrates to the core; After forging, remove the iron oxide scale and finish to the finished size of the mooring chain accessories.

[0021] A reasonable forging temperature range ensures that the steel has good plasticity, avoids overheating at high temperatures or brittle fracture at low temperatures, and further refines the grains during the forging process, thereby improving the overall mechanical properties of the accessories.

[0022] Compared with the prior art, the present invention has the following beneficial effects: 1) The “KR-BOF-LF-RH-CCM” smelting and continuous casting process is adopted to produce mooring chain forging accessories using R4 grade 4 mooring chain medium carbon high alloy MnCrNiMo steel. The steel is manufactured by continuous casting and rolling as intermediate billets. This meets the development trend of energy from shallow sea to deep sea. The use of electroslag ingots to produce large-size mooring chain forging accessories in the energy sector has been changed to the production of Φ200-350mm hot-rolled intermediate billets of mooring chain forging steel using smelting and continuous casting of large round billets and high-reduction rolling technology.

[0023] 2) By adopting the composite electromagnetic stirring technology of crystallizer M-EMS and end F-EMS, the internal quality of the continuously cast billet is improved. By controlling the rolling heating process and the rough rolling reduction, high-grade mooring chain forging accessory steel intermediate billets with excellent internal quality of Mn, Cr, Ni and Mo series are produced. It can replace the existing use of cast ingot electroslag remelting as intermediate billet R4 grade mooring chain forging accessory steel. The advantages of continuous casting and rolling replacing cast ingot electroslag ingot are obvious, and the effects of reducing energy consumption, low carbon and weight can be achieved. Attached Figure Description

[0024] Figure 1 This is the transverse low-magnification microstructure of the head of the intermediate billet of Φ350mm rolled round steel produced by continuous casting in Embodiment 1 of the present invention; Figure 2 This is the transverse low-magnification microstructure of the tail end of the intermediate billet of Φ350mm rolled round steel produced by continuous casting in Embodiment 2 of the present invention. Detailed Implementation

[0025] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0026] Example 1 Chemical composition (weight percentage): C=0.22%, Si=0.20%, Mn=1.40%, P=0.012%, S=0.005%, Cr=0.80%, Mo=0.40%, Ni=0.90%, Nb=0.03%, Cu=0.15%, H=0.9ppm, balance being Fe and unavoidable impurities.

[0027] Manufacturing process: Smelting: After KR pretreatment, S=0.004%, P is removed to 0.012% in BOF converter, composition is adjusted by LF refining, and H=0.9ppm after RH vacuum degassing; Continuous casting: superheat 32℃, casting speed 0.5m / min, composite electromagnetic stirring current 180A, continuous casting billet Φ700mm, slow cooling at 513℃ for 48h. Rolling: Heating temperature 1223℃, holding temperature for 15h, initial rolling temperature 1100℃, final rolling temperature 860℃, total reduction rate 71.4%, rolled into Φ350mm round steel, slow cooling at 552℃; Annealing: Hold at 551℃ for 12.5h, then cool in the furnace to 250℃ and air cool. Inspection: Ultrasonic testing grade 4 qualified; low magnification structure: S=1.0, R=1.0, C=1.0 grade; mechanical properties: ReH=860MPa, Rm=938MPa, elongation 21.0%, reduction of area 71%, yield strength ratio 0.917, KV2=163 / 157 / 153J at -20℃, hydrogen embrittlement Z1 / Z2=0.99; Forging: Heating temperature 1260℃, holding temperature for 5 hours, initial forging temperature 1240℃, final forging temperature 820℃, forging to form mooring chain shackles.

[0028] Example 2 Chemical composition (weight percentage): C=0.20%, Si=0.22%, Mn=1.35%, P=0.010%, S=0.003%, Cr=0.75%, Mo=0.38%, Ni=0.85%, Nb=0.025%, Cu=0.12%, H=0.7ppm, balance being Fe and unavoidable impurities.

[0029] Manufacturing process: Smelting: After KR pretreatment, S=0.003%; P is removed to 0.010% in BOF converter; composition is adjusted by LF refining; H=0.7ppm after RH vacuum degassing. Continuous casting: superheat 33℃, casting speed 0.45m / min, composite electromagnetic stirring current 170A, continuous casting billet Φ700mm, slow cooling at 517℃ for 49h. Rolling: Heating temperature 1227℃, holding temperature 14.8h, initial rolling temperature 1097℃, final rolling temperature 855℃, total reduction rate 71.4%, rolled into Φ350mm round steel, slow cooling at 556℃; Annealing: Hold at 553℃ for 12.2h, then cool in the furnace to 251℃ and air cool. Inspection: Ultrasonic testing grade 4 qualified; low magnification structure S=1.0 grade, R=1.0 grade, C=1.0 grade; mechanical properties: ReH=850MPa, Rm=936MPa, elongation 20.0%, reduction of area 68%, yield strength ratio 0.908, KV2=157 / 163 / 153J at -20℃, hydrogen embrittlement Z1 / Z2=1.00; Forging: Heating temperature 1250℃, holding temperature 4.5h, initial forging temperature 1230℃, final forging temperature 810℃, forging to form mooring chain swivel.

[0030] Comparative test R4 grade IV mooring chain accessory steel produced by the existing electroslag remelting process was selected as a control group and compared with the products of Examples 1 and 2 of this invention in terms of performance and production indicators. The results are as follows:

[0031] The comparison results show that the mechanical properties of the product of this invention are better than those of the control group, and the production energy consumption, cycle and cost are significantly reduced, which fully demonstrates the technical advantages of this invention.

[0032] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A steel for anchor chain mooring chain accessory forging, characterized in that, The chemical composition in percentage by weight is: C: 0.18-0.28%, Si: 0.15-0.30%, Mn: 1.00-1.80%, P: ≤0.015%, S: ≤0.010%, Cr: 0.40-1.30%, Mo: 0.20-0.60%, Ni: 0.40-1.40%, Nb: 0.01-0.06%, Cu: ≤0.20%, H: ≤1.5ppm, and the balance being iron and inevitable impurities.

2. A steel for anchor chain mooring chain accessory forging according to claim 1, characterized in that, The macrostructure of the steel satisfies S≤2.0, R≤2.0, C≤2.0 according to ASTM E381 standard, the mechanical properties satisfy ReH≥580MPa, Rm≥860MPa, elongation≥12%, area reduction≥50%, yield ratio≤0.92, -20℃KV2≥80J, and hydrogen embrittlement Z1 / Z2≥0.

85.

3. A method of manufacturing a steel for forging a mooring chain accessory of an anchor chain as claimed in claim 1, characterized in that, The method comprises the following steps: 1) Steel smelting: KR pretreatment-BOF converter smelting-LF refining-RH vacuum degassing composite process, the molten iron is treated by KR pretreatment, then P and C are removed in the BOF converter, the composition is adjusted after tapping by LF refining, and then RH vacuum degassing treatment is performed, so that P≤0.015%, S≤0.010%, and H≤1.5ppm are controlled at tapping; 2) Continuous casting: argon protection is used during the whole process, M-EMS and F-EMS are used for composite electromagnetic stirring, the superheat is controlled to be 20-35℃, the casting speed is controlled to be 0.4-0.6m / min, and the continuous casting blank is discharged at≥500℃ and then enters the slow cooling pit for heat preservation for≥40h; 3) Rolling: the casting blank is heated to 1220-1270℃ at a heating rate of≤60℃ / h, heat preservation is performed for 14-16h, the opening rolling temperature is 950-1100℃, the final rolling temperature is≥850℃, and the Φ200-350mm round steel is formed through large reduction multi-stand continuous rolling, and then the round steel is discharged at≥300℃ and slowly cooled to≤200℃; 4) Stress relief annealing: the annealing temperature is 520-580℃, heat preservation is performed for 12-20h, the furnace is cooled to≤250℃, and then air cooling is performed to room temperature; 5) Inspection: EN 10228-3 standard 4-level ultrasonic flaw detection is performed on each piece, and the macrostructure and mechanical properties are detected according to batches; 6) Forging: the intermediate rolling blank is heated to 1240-1280℃ at a heating rate of≤200℃ / h, heat preservation is performed for 4-6h, the initial forging temperature is≤1250℃, and the final forging temperature is≥800℃, and then the intermediate rolling blank is forged into a shape.

4. The production method according to claim 3, characterized by In step 1), the vacuum degree of the RH vacuum degassing treatment is≤67Pa, and the degassing time is≥20min.

5. The production method according to claim 3, wherein In step 2), the current of the composite electromagnetic stirring is 150-200A, and the frequency is 2-4Hz.

6. The production method according to claim 3, characterized by In step 3), the total reduction rate of rolling is≥70%, and the reduction rate is controlled in three stages of initial rolling, intermediate rolling and finish rolling, the initial rolling reduction rate is 30-40%, the intermediate rolling reduction rate is 35-45%, and the finish rolling reduction rate is 15-25%.

7. The production method according to claim 3, wherein In step 3), high-pressure water is used for descaling during the rolling process, and the water pressure is≥18MPa.

8. The production method according to claim 3, wherein In step 6), the mooring chain accessories after forging include enlarged chain rings, end chain rings, shackles or rotating rings.

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