A steel for oil and gas forgings and a method for producing the same

Abstract

The application discloses an oil and gas forging steel and a production method thereof. The chemical components of the oil and gas forging steel are as follows in percentage by weight: C: 0.15-0.20%, Si: 0.15-0.35%, Mn: 0.50-0.70%, Cr: 2.20-2.60%, Mo: 0.90-1.20%, Ni: 0.20-0.40%, Al: 0.020-0.040%, Ti: 0.0015-0.0045%, B: 0.001-0.003%, P: <=0.012%, S: <=0.003%, T.O: <=15ppm, [N]: <=70ppm, [H]: <=1.5ppm, and the rest is Fe and other inevitable impurities. The oil and gas forging steel contains nanoscale titanium carbide particles, has good strength and toughness matching and low-temperature performance, and after being processed into a forging, the austenite grain size at the center of the wall thickness of the forging is greater than or equal to 9.0.

Description

Technical Field

[0001] This invention belongs to the field of steel technology, specifically relating to a steel for oil and gas forgings and its production method. Background Technology

[0003] Key forgings in deep-sea oil and gas wellhead control systems, such as valve bodies and flanges, need to possess ultra-high strength, excellent low-temperature toughness, and resistance to H2S / CO2 corrosion.

[0004] Low-carbon Cr-Mo steel is commonly used for high-performance oil and gas forgings, but it has significant limitations for sections thicker than 150mm: peritectic reactions easily occur during continuous casting, leading to core defects; Cr and Mo segregation forms segregation bands, resulting in the precipitation of coarse Mo during heat treatment. 23 C6 carbides cause localized embrittlement; the thickness of forgings is usually over 200mm, resulting in a significant wall thickness effect, poor microstructure uniformity, and performance at 1 / 4 wall thickness is difficult to meet standards; at the same time, due to the high Cr and Mo content of this material (Cr+Mo≥3.2%), after slow cooling annealing, a small number of central cracks still exist in the core of the continuously cast billet, which can easily lead to non-compliance in flaw detection during subsequent forging processes.

[0005] For example, the AISI 8630 modified steel, widely used in deep-sea oil and gas wellhead forgings, has a typical chemical composition of: C 0.18%, Si 0.30%, Mn 0.65%, Cr 2.40%, Mo 1.00%, Ni 0.35%, with the balance being Fe and unavoidable impurities. When this steel is used to produce continuously cast round billets with a diameter of φ600mm or more using conventional continuous casting processes, and then forged into valve bodies or flange forgings with a wall thickness exceeding 200mm, the ultrasonic flaw detection pass rate of the forgings is typically only 85-90%. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention provides a steel for oil and gas forgings, wherein the steel contains nano-sized titanium carbide particles, and has good strength and toughness matching and low-temperature performance.

[0007] The present invention also provides a method for producing steel continuous casting round billets for oil and gas forgings, wherein the φ600~φ900mm continuous casting round billets produced by the method have a low magnification rating of center porosity ≤1.0.

[0008] This invention also provides an oil-gas forging, which is obtained by forging, rough turning, hole turning, heat treatment, rough turning, UT, and finish turning of a continuously cast round billet produced by the production method described in this invention. The oil-gas forging has a tensile strength of 680~810MPa, a yield strength of 550~680MPa, an elongation after fracture ≥20%, a reduction of area ≥60%, an impact energy of -59℃ ≥180J, a sum of non-metallic inclusions of types A, B, C, and D ≤2.5, and an austenite grain size of grade ≥9.0 at the center of the forging wall thickness.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] This invention provides a steel for oil and gas forgings, the chemical composition of which, by weight percentage, is: C: 0.15-0.20%, Si: 0.15-0.35%, Mn: 0.50-0.70%, Cr: 2.20-2.60%, Mo: 0.90-1.20%, Ni: 0.20-0.40%, Al: 0.020-0.040%, Ti: 0.0015-0.0045%, B: 0.001-0.003%, P: ≤0.012%, S: ≤0.003%, TO: ≤15ppm, [N]: ≤70ppm, [H]: ≤1.5ppm, with the remainder being Fe and other unavoidable impurities;

[0011] The chemical composition of the steel used for oil and gas forgings satisfies: X = 1.2 × C + 0.6 × Mn + 0.5 × Ni + 0.7 × Cr + 0.8 × Mo, 3.00 ≤ X ≤ 3.45; in the formula, the value of each chemical component is its content in the steel × 100.

[0012] The metallographic structure of the steel used for oil and gas forgings is tempered sorbite; the steel used for oil and gas forgings contains nano-sized titanium carbide particles.

[0013] The steel used for oil and gas forgings has a tensile strength of 680~810MPa, a yield strength of 550~680MPa, an elongation after fracture ≥20%, a reduction of area ≥60%, an impact energy of -59℃ ≥180J, a sum of non-metallic inclusions of types A, B, C, and D ≤2.5, and an austenite grain size at the center of the forging wall thickness ≥9.0 grade; the low-magnification rating of the center porosity of the φ600~φ900mm continuous casting round billet is ≤1.0 grade.

[0014] The present invention also provides a method for producing the steel continuous casting round billet for oil and gas forgings, the method comprising the following steps: hot metal pretreatment → converter smelting → LF furnace refining → RH vacuum degassing → round billet continuous casting → annealing.

[0015] In the molten iron pretreatment step: the molten iron S≤0.035%, it is only processed at the slag removal station, and the bright surface of the slag removal is ≥70%.

[0016] In the converter smelting process, the steel is deoxidized and alloyed after tapping, while the amount of slag is strictly controlled, and the smelting cycle is 30-40 minutes.

[0017] In the LF furnace refining step, elements such as C, Si, Mn, Cr, Ni, and Mo are adjusted to target values, the white slag retention time is ≥20 min, and the smelting cycle is ≤80 min.

[0018] In the RH vacuum degassing step, during the later stage of vacuum degassing, 30-60m of cored wire is fed in, with each meter of cored wire containing 380-510g of ferrotitanium (FeTi) powder. 30 ) and 30-40g of graphite powder, using the strong stirring and high temperature of the RH device, to make titanium and carbon react in situ inside the molten steel to generate nano-sized TiC particles.

[0019] In the RH vacuum degassing step, argon is used as the booster gas, with a gas flow rate of 500~1000L / min, a vacuum degree of ≤100Pa, and a holding time of 20~30min; the temperature of the RH device is controlled at 1570℃~1605℃.

[0020] In the RH vacuum degassing step, after feeding in 30-60m of cored wire, soft blowing is performed for 5-15 minutes, followed by feeding in 40-70m of calcium wire and continuing soft blowing for 15-25 minutes; the soft blowing flow rate is 10-150L / min; after vacuum treatment, [H] ≤ 1.0ppm.

[0021] In the continuous casting process of the round billet, the superheat of the tundish is controlled at 30-40°C above the liquidus line; an immersion nozzle and protective slag are used for casting to prevent secondary oxidation and absorb inclusions floating in the molten steel; the specific water volume in the secondary cooling zone of the continuous casting machine is 0.3L / t~0.4L / t. If the specific water volume is too high, it will easily lead to overcooling of the billet surface, thereby causing surface cracks, center segregation, or bulging deformation; if the specific water volume is too low, the billet solidifies too slowly, which easily leads to internal porosity and shrinkage cavities; the continuous casting speed is 0.16~0.25m / min. Using a low casting speed can prolong the residence time of the billet in the secondary cooling zone, allowing the molten steel at the solidification front more time to feed, reducing internal defects such as center segregation, shrinkage cavities, and porosity.

[0022] In the straightening step, straightening is performed when the surface temperature of the casting drops to above 900°C, which is above the brittle temperature zone. The temperature range of 700~900°C is the brittle temperature zone of the casting. If the surface temperature of the casting is within this range, transverse cracks are easily generated during straightening. The present invention controls the water volume in the secondary cooling zone of the continuous casting machine to be 0.3L / t~0.4L / t, and keeps the surface temperature of the casting above 900°C by controlling the intensity of secondary cooling.

[0023] In the annealing step, the continuously cast round billet is fed into the annealing furnace at a temperature above 400℃, heated to 650±10℃ at a rate of ≤40℃ / h, held at that temperature, and then air-cooled to room temperature. The holding time is t=0.04d-12, where t is in hours and d is the diameter of the continuously cast round billet in mm. The annealing step can release the internal stress accumulated in the steel during the continuous casting cooling process, preventing deformation and cracking of the steel due to excessive internal stress.

[0024] The present invention also provides an oil and gas forging, which is obtained by processing a continuously cast round billet produced by the production method described in the present invention through forging, rough turning, hole turning, heat treatment, rough turning, ultrasonic testing, and finish turning.

[0025] The forging process involves three-stage forging with a forging ratio ≥5; the heat treatment steps are: quenching at 850℃~950℃, tempering at 600~750℃, and then air cooling to room temperature.

[0026] The functions and control of each component in the oil and gas forging steel provided by this invention are as follows:

[0027] C: As a core strengthening element, carbon strengthens through solid solution and forms carbides such as Fe3C and M. 23 C6 increases strength; each 0.1% increase raises the yield strength by 60-80 MPa. It also lowers the martensitic transformation temperature (Ms point) and significantly increases hardenability. However, excessive carbon leads to an increase in carbon equivalent (CEV), which worsens weldability and raises the ductile-brittle transition temperature. This invention controls C ≤ 0.20% to balance strength and toughness, thus determining the C content range to be 0.15-0.20%.

[0028] Si: Solid solution strengthens ferrite, increasing strength by 10 MPa for every 0.1% Si, and assists in deoxidation to form low-melting-point SiO2-MnO composite inclusions. It inhibits cementite spheroidization, but >0.4% will promote high-temperature ferrite and reduce toughness, so the Si content is controlled at 0.15~0.35%.

[0029] Mn: It enhances strength through solid solution strengthening and expands the γ-phase region to stabilize austenite. Its unique role lies in lowering the ductile-brittle transition temperature; every 0.1% Mn increases the impact energy at -60℃ by 5~10J. Furthermore, it combines with sulfur to form MnS with a high melting point of 1600℃, eliminating the red brittleness caused by FeS. This invention controls the Mn content to be within the range of 0.50~0.70%.

[0030] Cr: A core corrosion-resistant element, forming a dense Cr2O3 passivation film of 5-10 nm on the steel surface, reducing CO2 corrosion rate. In terms of hardenability, it strongly inhibits ferrite nucleation; every 0.1% Cr increases the critical diameter by 4-6 mm. It also improves high-temperature strength, but its content must be controlled to ≤2.6% to avoid σ-phase precipitation and embrittlement; therefore, the Cr content is controlled between 2.20% and 2.60%.

[0031] Mo significantly delays the bainite transformation, with a hardenability enhancement twice that of Cr. It induces secondary hardening through Mo₂C formation and inhibits P / S grain boundary segregation, reducing the temper brittleness temperature by more than 15°C. However, too low a Mo content is insufficient to exert its effect, while too high a content affects the hot plasticity of the steel, increasing the risk of reheat cracking and raising costs. Therefore, the Mo content is controlled at 0.90~1.20%.

[0032] Ni: The only element that significantly improves low-temperature toughness while increasing strength: every 0.1% Ni lowers the ductile-brittle transition temperature by approximately 6-8°C and increases the impact energy at -60°C by 8-12 J. Its mechanism of action is to stabilize austenite and increase stacking fault energy, promoting ductile fracture morphology. It also inhibits hydrogen permeation in acidic environments, increasing the H2S stress corrosion threshold (KISSC) by 5%. However, due to Ni's high cost and the fact that excessive content can reduce machinability after hot working, the Ni content is controlled between 0.20% and 0.40%.

[0033] Ti: Adding Ti-containing cored wire to molten steel generates TiC particles at high temperatures. These TiC particles act as highly efficient heterogeneous nucleation sites, significantly increasing the nucleation rate. This means a dramatic increase in the number of crystal nuclei per unit volume, ultimately resulting in a significantly refined solidification structure. After grain refinement, the dendrite spacing decreases significantly. The migration path (from the dendrite trunk to the interdendritic space) required for solute elements becomes very short. The formation of fine and uniform equiaxed crystal regions prevents the growth of coarse columnar crystals, which are the main cause of central negative segregation and V-type segregation. The fine equiaxed crystal regions make it difficult for solutes to migrate and accumulate over long distances, effectively suppressing the segregation of elements such as C, Mn, and Cr. Furthermore, nanoparticles can pin grain boundaries, inhibiting the growth of austenite grains at high temperatures, ultimately improving the overall mechanical properties of the material. However, excessive addition can clog the gate and affect casting; therefore, the Ti content is controlled between 0.0015% and 0.0045%.

[0034] B: Trace amounts of boron are the most significant element in improving the hardenability of steel. The core mechanism lies in the segregation of dissolved boron atoms at the austenite grain boundaries, strongly inhibiting the nucleation of proeutectoid ferrite. This ensures a uniform Marbe microstructure in the core of thick-section forgings under slow cooling conditions, effectively solving the problem of "performance failing to meet standards at 1 / 4 of the wall thickness." However, boron readily combines with residual nitrogen in the steel to form coarse BN particles, leading to a complete loss of hardenability. This invention pre-adds trace amounts of Ti to preferentially fix nitrogen, protecting the activity of dissolved boron and achieving a Ti-B synergistic microalloying design. However, too low a boron content results in insufficient hardenability improvement, while too high a content will exceed the solid solubility limit, causing Fe to precipitate at the grain boundaries. 23 Boron-containing brittle phases such as (C,B)6 or Fe2B drastically worsen grain boundary strength, inducing "boron embrittlement" and increasing the risk of hot working cracks. Therefore, the B content should be controlled at 0.001~0.003%.

[0035] Al: Al is a highly efficient deoxidizer. It combines with oxygen to form Al2O3 inclusions, which then float to the surface, resulting in TO ≤ 15ppm and improving the cleanliness of the steel. The grain refinement effect is achieved by AlN pinning austenite grain boundaries, inhibiting grain growth at temperatures > 950℃. Controlling the Al content avoids the formation of clustered Al2O3, reducing the defect rate during flaw detection. This invention controls the Al content at 0.020-0.040%.

[0036] P and S: Sulfur readily combines with manganese in steel to form MnS inclusions, causing hot brittleness. However, adding a small amount of S significantly improves the machinability of steel without affecting product performance. MnS also has a grain-refining effect. P is an element with a strong tendency to segregate, increasing cold brittleness and reducing plasticity in steel, which is detrimental to the uniformity of product microstructure and properties. Control P ≤ 0.012% and S ≤ 0.003%.

[0037] TO and [H]: TO is the main cause of inclusions and internal oxidation in steel. Strictly controlling the oxygen content in steel is the key to ensuring the performance of steel used in oil and gas forgings. The accumulation of hydrogen in steel will lead to a significant reduction in the material's plasticity, toughness, and fatigue strength, and in severe cases, it will cause cracks or brittle fracture. Therefore, TO ≤ 15ppm and [H] ≤ 1.5ppm.

[0038] [N]: [N] forms compounds with Al and other substances that can refine grains. A reasonable Al / [N] ratio has a significant effect on grain refinement. However, excessive [N] can cause continuous casting defects, so the [N] content is controlled at ≤70ppm.

[0039] To ensure the steel possesses a good balance of strength and toughness, as well as corrosion resistance, the proportions of Cr, Ni, Mo, C, and Mn must be carefully controlled. Since C strengthens through solid solution and forms carbides, improving the hardenability and strength of the steel, its coefficient is 1.2. Because Cr improves the hardenability of the material, forms fine carbides, and increases the strength of the steel, while also forming a passivation film to improve corrosion resistance, its coefficient is 0.7. Mn strengthens through solid solution and stabilizes austenite, improving hardenability and lowering the ductile-brittle transition temperature, thus improving low-temperature toughness, its coefficient is 0.6. Ni improves toughness and low-temperature performance, reduces quenching crack sensitivity, and increases resistance to acidic environments, its coefficient is 0.5. Mo refines grain size, improves the hardenability of the material, forms stable carbides with carbon, enhances resistance to plastic deformation, and improves the strength, toughness, and corrosion resistance of the steel, its coefficient is 0.8.

[0040] X=1.2×C+0.6×Mn+0.5×Ni+0.7×Cr+0.8×Mo, 3.00≤X≤3.45.

[0041] The present invention has the following beneficial effects:

[0042] In the production method of continuously cast round billets for oil and gas forgings provided by this invention, Ti-containing cored wire is added during the RH vacuum degassing process of steelmaking, generating TiC nanoparticles in situ within the steel. During continuous casting, this effectively inhibits dendrite growth, reduces elemental segregation, and refines the grain structure, increasing the forging flaw detection pass rate to ≥99.5%. As heterogeneous nucleation cores, TiC nanoparticles significantly increase the nucleation rate, refine the solidification structure, reduce dendrite spacing, inhibit elemental segregation, and promote the formation of uniform and fine equiaxed grains, thereby reducing central segregation. Furthermore, the nanoparticles can pin grain boundaries, inhibit austenite grain growth, and comprehensively improve the mechanical properties and uniformity of the material.

[0043] The oil and gas forgings produced by the production method provided by this invention are manufactured from continuously cast steel round billets through forging, rough turning, hole turning, heat treatment, rough turning, ultrasonic testing, and finish turning. The resulting oil and gas forgings have a metallographic structure of tempered sorbite, a tensile strength of 680~810MPa, a yield strength of 550~680MPa, an elongation after fracture ≥20%, a reduction of area ≥60%, an impact energy of -59℃ ≥180J, a sum of non-metallic inclusions of types A, B, C, and D ≤2.5, and an austenite grain size of ≥9.0 grade at the center of the forging wall thickness. Attached Figure Description

[0044] Figure 1 The image shows the metallographic structure of the steel used for oil and gas forgings in Example 1. The metallographic structure is tempered sorbite.

[0045] Figure 2 The image shows a SEM image of the steel used for oil and gas forgings in Example 1, which contains nano-sized TiC particles. Detailed Implementation

[0046] The present invention provides a steel for oil and gas forgings, the chemical composition of which, by weight percentage, is: C: 0.15-0.20%, Si: 0.15-0.35%, Mn: 0.50-0.70%, Cr: 2.20-2.60%, Mo: 0.90-1.20%, Ni: 0.20-0.40%, Al: 0.020-0.040%, Ti: 0.0015-0.0045%, B: 0.001-0. 0.003%, P: ≤0.012%, S: ≤0.003%, TO: ≤15ppm, [N]: ≤70ppm, [H]: ≤1.5ppm, the remainder being Fe and other unavoidable impurities; its chemical composition satisfies: X=1.2×C+0.6×Mn+0.5×Ni+0.7×Cr+0.8×Mo, 3.00≤X≤3.45; in the formula, the value of each chemical component is its content in the steel ×100.

[0047] The production method of the steel for oil and gas forgings is as follows:

[0048] The production of round billets through continuous casting includes the following steps: hot metal pretreatment → converter smelting → LF furnace refining → RH vacuum degassing → round billet continuous casting → annealing; specifically as follows:

[0049] Hot metal pretreatment: Hot metal S≤0.035%, treated only at the slag removal station, with a slag removal bright surface ≥70%.

[0050] Converter: The steel is deoxidized and alloyed after tapping, while the amount of slag is strictly controlled. The smelting cycle is 30-40 minutes.

[0051] LF furnace refining: C, Si, Mn, Cr, Ni, Mo and other elements are adjusted to the target values, white slag retention time is ≥20min, and smelting cycle is ≤80min.

[0052] RH vacuum degassing: Argon is used as the lifting gas, with a flow rate of 500~1000 L / min, a vacuum degree ≤100 Pa, and a holding time of 20~30 min. In the later stage of vacuum degassing, 30~60 m of cored wire is fed into the molten steel using a wire feeder. Each meter of cored wire contains 380~510 g of ferrotitanium powder and 30~40 g of graphite powder. The RH device temperature is controlled at 1570℃~1605℃. The strong stirring and high temperature of the RH device allow titanium and carbon to react in situ within the molten steel, generating nano-sized TiC particles. After feeding the cored wire, soft blowing continues for 5~15 min. Subsequently, 40~70 m of Ca wire is fed into the steel using a wire feeder, followed by soft blowing for 15~25 min. The argon flow rate for soft blowing is 10-150 L / min, and the total smelting cycle is controlled at 80~120 min. After vacuum treatment, [H] ≤1.0 ppm.

[0053] Continuous casting: The superheat of the tundish is controlled at 30~40℃ above the liquidus line. Immersion nozzles with protective slag are used for casting. The water volume in the secondary cooling zone of the continuous casting machine is 0.3L / t~0.4L / t. The casting speed is 0.16~0.25m / min. Straightening is performed when the surface temperature of the casting is reduced to above 900℃.

[0054] Annealing: The continuously cast round billet is fed into an annealing furnace at a temperature above 400℃, heated to 650±10℃ at a rate of ≤40℃ / h, held at that temperature, and then air-cooled to room temperature; the holding time is t=0.04d-12, where t is in hours and d is the diameter of the continuously cast round billet in mm.

[0055] The production of oil and gas forgings includes the following steps: continuous casting round billet → rough turning → hole turning → heat treatment → rough turning → ultrasonic testing → finish turning. Specifically: the forging process involves three-stage forging with a forging ratio ≥ 5; the heat treatment steps are: quenching at 850℃~950℃, tempering at 600~750℃ followed by air cooling to room temperature. The tempering holding time t is determined according to the maximum wall thickness D (unit: mm) of the forging using the following formula: t = 0.04D + 2, where t is in hours (h).

[0056] The present invention will now be described in detail with reference to the embodiments.

[0057] The chemical composition and weight percentage of the steel used for oil and gas forgings in each embodiment and comparative example are shown in Table 1.

[0058] Table 1 Chemical Composition

[0059]

[0060] In Table 1, the units for [H], TO, and [N] are ppm, and the units for other chemical components are wt%.

[0061] The steel with the composition shown in Table 1 is produced by smelting molten iron through the steps of hot iron pretreatment → converter smelting → LF furnace refining → RH vacuum degassing, and then continuously cast round billets are produced by round billet continuous casting → annealing.

[0062] The main production process parameters for the production of continuously cast round billets for oil and gas forgings in each embodiment and comparative example are shown in Table 2.

[0063] Table 2

[0064]

[0065] The continuously cast round billets obtained from each embodiment and comparative example were sawn into 40mm low-magnification slices. According to standard GB / T 226, the test method was acid leaching, and the rating chart in Appendix D of YB / T 153 was used for evaluation. The results are shown in Table 3.

[0066] Table 3

[0067]

[0068] The continuously cast round billets obtained from the above embodiments and comparative examples are processed into forgings by rough turning → hole turning → heat treatment → rough turning → ultrasonic testing → finish turning. The main process parameters are shown in Table 4.

[0069] Table 4

[0070]

[0071] Mechanical property tests were conducted at 1 / 2 wall thickness of the forgings according to GB / T228 and GB / T229, and the results are shown in Table 5. Non-metallic inclusions were inspected and rated according to ASTM E45 A method, and the results are shown in Table 6. Austenite grain size was inspected according to GB / T 6394 direct hardening method, and the results are shown in Table 5.

[0072] Table 5

[0073]

[0074] Table 6

[0075]

[0076] As can be seen from the above, since the chemical composition and production method of the steel in Examples 1 to 3 are appropriately controlled in accordance with the requirements of the present invention, their chemical composition meets the requirements, and the mechanical properties, non-metallic inclusions, and austenite grain size of the steel are all good.

[0077] In Comparative Examples 1 and 2, although the content of each component was controlled according to the requirements of the present invention, the components did not meet the requirement of 3.00≤X=1.2×C+0.6×Mn+0.5×Ni+0.7×Cr+0.8×Mo≤3.45, resulting in unsatisfactory overall performance.

[0078] In Comparative Example 3, the cored wire containing titanium iron powder and graphite powder was not fed during the RH vacuum treatment process, which prevented the formation of TiC nano-ceramic particles, resulting in unsatisfactory overall performance.

[0079] The above detailed description of a steel for oil and gas forgings and its production method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments may be listed within the defined scope. Therefore, variations and modifications that do not depart from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A type of steel for oil and gas forgings, characterized in that, The chemical composition of the steel used for oil and gas forgings, by weight percentage, is as follows: C: 0.15-0.20%, Si: 0.15-0.35%, Mn: 0.50-0.70%, Cr: 2.20-2.60%, Mo: 0.90-1.20%, Ni: 0.20-0.40%, Al: 0.020-0.040%, Ti: 0.0015-0.0045%, B: 0.001-0.003%, P: ≤0.012%, S: ≤0.003%, TO: ≤15ppm, [N]: ≤70ppm, [H]: ≤1.5ppm, with the remainder being Fe and other unavoidable impurities; The chemical composition of the steel used for oil and gas forgings satisfies: X = 1.2 × C + 0.6 × Mn + 0.5 × Ni + 0.7 × Cr + 0.8 × Mo, 3.00 ≤ X ≤ 3.

45.

2. The steel for oil and gas forgings according to claim 1, characterized in that, The metallographic structure of the steel used for oil and gas forgings is tempered sorbite; the steel used for oil and gas forgings contains nano-sized titanium carbide particles.

3. The steel for oil and gas forgings according to claim 1, characterized in that, The steel used for oil and gas forgings has a tensile strength of 680~810MPa, a yield strength of 550~680MPa, an elongation after fracture ≥20%, a reduction of area ≥60%, an impact energy of -59℃ ≥180J, a sum of non-metallic inclusions of types A, B, C, and D ≤2.5, and an austenite grain size at the center of the forging wall thickness ≥9.0 grade; the low-magnification rating of the center porosity of the φ600~φ900mm continuous casting round billet is ≤1.0 grade.

4. The method for producing continuously cast round billets of steel for oil and gas forgings as described in any one of claims 1-3, characterized in that, The production method includes the following steps: hot metal pretreatment → converter smelting → LF furnace refining → RH vacuum degassing → round billet continuous casting → annealing; In the RH vacuum degassing step, during the later stage of vacuum degassing, 30-60m of cored wire is fed in, with each meter of cored wire containing 380-510g of ferrotitanium powder and 30-40g of graphite powder.

5. The method for producing continuously cast round billets of steel for oil and gas forgings according to claim 4, characterized in that, In the RH vacuum degassing step, argon is used as the booster gas, with a gas flow rate of 500~1000L / min, a vacuum degree of ≤100Pa, and a holding time of 20~30min; the temperature of the RH device is controlled at 1570℃~1605℃.

6. The method for producing continuously cast round billets of steel for oil and gas forgings according to claim 4, characterized in that, In the RH vacuum degassing step, after feeding in 30-60m of cored wire, soft blowing is performed for 5-15 minutes, followed by feeding in 40-70m of calcium wire and continuing soft blowing for 15-25 minutes; the soft blowing flow rate is 10-150L / min; after vacuum treatment, [H] ≤ 1.0ppm.

7. The method for producing continuously cast round billets of steel for oil and gas forgings according to claim 4, characterized in that, In the continuous casting step of the round billet, the superheat of the tundish is controlled at 30~40°C above the liquidus line; an immersion nozzle and protective slag are used for casting; the water volume in the secondary cooling zone of the continuous casting machine is 0.3L / t~0.4L / t; and the continuous casting speed is 0.16~0.25m / min.

8. The method for producing continuously cast round billets of steel for oil and gas forgings according to claim 4, characterized in that, In the annealing step, the continuously cast round billet is fed into the annealing furnace at a temperature above 400°C, heated to 650±10°C at a rate of ≤40°C / h, held at that temperature, and then air-cooled to room temperature; the holding time is t=0.04d-12, where t is in hours and d is the diameter of the continuously cast round billet in mm.

9. An oil and gas forging, characterized in that, The continuously cast round billet produced by the production method according to any one of claims 4-8 is obtained by forging, rough turning, hole turning, heat treatment, rough turning, ultrasonic testing, and finish turning.

10. The oil and gas forging according to claim 9, characterized in that, The forging process involves three-stage forging with a forging ratio ≥5; the heat treatment steps are: quenching at 850℃~950℃, tempering at 600~750℃, and then air cooling to room temperature.

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