A method for manufacturing high-strength and toughness casing

Abstract

The present invention discloses a method for manufacturing a high-strength and toughness casing, which belongs to the field of steel materials for oil and gas. The chemical composition of the casing has specific weight percentages of multiple elements such as C, Si, and Mn, and is made through multiple processes such as molten iron pretreatment and converter smelting. During the manufacturing process, the process parameters of each link are strictly controlled, such as RH degassing time, temperature of each stage, annealing and holding time, etc. The raw steel coil after pre-annealing has low strength, good plasticity and toughness, is easy to form, and the banded structure is ≤ level 2. After welding, thermal tension reduction and full pipe heat treatment, the finished pipe has Rp 0.2 ≥900MPa, R m ≥985MPa, elongation ≥22%, longitudinal impact energy of the parent material at ‑10°C ≥130J, and a homogenized tempered bainite structure. This invention addresses the challenges of high-strength and toughness casing raw materials, including the difficulty of rolling and cracking during welding, and meets the stringent requirements for casing performance in unconventional oil and gas production.

Description

Technical Field

[0001] The present invention relates to the technical field of steel materials for petroleum and natural gas, and in particular to a method for manufacturing a high-strength and toughness casing. Background Art

[0002] With the massive depletion of conventional oil and gas resources, the exploitation of unconventional oil and gas resources, such as shale gas and tight oil, is increasing year by year. Unconventional oil and gas fields have complex reservoir environments, and multi-stage volumetric fracturing and dense cutting technology has become an effective means of increasing reserves and production. However, this fracturing operation places increasingly stringent requirements on the strength, toughness, and collapse resistance of oil and gas casing. Consequently, demand for high-strength, high-toughness, large-diameter casing of Q125 and above is increasing.

[0003] High-strength, high-toughness casing is manufactured from hot-rolled coils through a series of processes, including slitting, roll forming, HFI welding, in-line weld seam heat treatment, weld seam extrusion sizing, and full-tube heat treatment. High-strength grades, such as Q125, can achieve yield strengths exceeding 900 MPa. While post-quenching and tempering treatment is crucial for enhancing strength, the strength design and control of the raw coil must be kept within a reasonable range, as this will negatively impact the mechanical and fatigue properties of the finished pipe. Microalloying with carbon (>0.20%), combined with niobium, vanadium, and titanium (V), is commonly employed. However, this high-strength raw coil can be difficult to roll and coil. Furthermore, casing steels often use medium carbon and manganese content, resulting in a more pronounced banded structure in the hot-rolled raw coil. This increases the risk of cracking near the weld during SEW welding and compromises overall properties such as HIC resistance. Therefore, the key challenge is to achieve low raw coil strength while ensuring ease of forming and welding during pipemaking, while maintaining good strength and ductility after quenching and tempering. This is a technical contradiction and difficulty in the whole process of manufacturing high-strength and toughness casing.

[0004] At present, a number of patents have been applied for in China regarding high-strength and toughness casing and its manufacturing method.

[0005] Patent publication number CN103194693A discloses a high-strength, high-toughness oil casing and its manufacturing method. Its chemical composition is C: 0.18%-0.30%, Si: 0.10%-0.35%, Mn: 1.00%-1.30%, P: ≤0.018%, S: ≤0.008%, Cr: 0.20%-1.20%, Ni: 0.10%-0.65%, Mo: 0.20%-0.55%, and Nb+V+Ti ≤0.08%. The casing has a yield strength of ≥865 MPa, a tensile strength of ≥940 MPa, and an elongation of ≥23.2%. The raw material coil is directly slit, milled, FFX-formed, HFI-welded, heat-stretched, and fully heat-treated. The patent does not provide technical specifications for the raw material coil's banded structure or mechanical properties, nor does it describe how to control the difficulty of forming. This patented design utilizes a high-Mo (0.20-0.55%) and Cr (0.20-1.20%) alloy, combined with HFW welding, online heat treatment, and quenching and tempering. The finished tube achieves a yield strength of ≥865 MPa, a tensile strength of ≥940 MPa, and an impact energy of ≥105 J (transverse direction) at 0°C. However, challenges include unoptimized raw coil strength, insufficient control of banded structure, and high alloy costs.

[0006] Patent publication number CN112522607A discloses Q125 steel-grade SEW oil casing and its manufacturing method. Its chemical composition is C: 0.15%-0.25%, Si: ≤0.30%, Mn: 1.00%-1.50%, P: ≤0.015%, S: ≤0.005%, Mo: 0.10-0.50%, V: 0.03-0.06%, Ti: 0.01-0.03%, Cr: 0.20-0.60%, Als: 0.02-0.035%. The application describes hot-rolled coils produced at 650-750°C, with a yield strength of 400-500 MPa and a tensile strength of 600-700 MPa. Subsequent SEW welding and forming are followed by quenching and tempering. The resulting coils are 905-950 MPa in yield strength and 990-1100 MPa in tensile strength. The casing described in this patent is not preheated before full-tube quenching and tempering. The yield strength difference between the raw coil and the finished pipe is small (approximately 450 MPa), and the impact energy of the finished pipe at -10°C is only 85-98 J. The patent also fails to address the reduction of banded structure. This patent utilizes high-temperature coiling (650-750°C) combined with SEW pipemaking and quenching and tempering to reduce the Mo content (0.10-0.50%). The technical effect is a raw coil yield strength of 400-500 MPa and a finished pipe impact energy ≥80 J (transversely at -10°C). However, the raw coil strength remains relatively high, the low-temperature toughness is insufficient, and HIC resistance is unclear.

[0007] Therefore, the challenge is to design and develop a high-strength and tough casing with consistent microstructure and performance throughout the entire tube, easy forming, and weldability. This invention addresses multiple aspects, including key component design, steelmaking and rolling, pre-annealing of the raw coil, tube forming, and full-tube heat treatment, to achieve the production of a high-strength and tough casing with annealed raw coil, good microstructure uniformity, easy forming, and weldability. Summary of the Invention

[0008] To address the problems of the prior art, the present invention provides a method for manufacturing high-strength and high-toughness casing. The hot-rolled raw coil of the casing, after pre-annealing, exhibits low strength, good plasticity and toughness, and is easily formable. The banded structure is well-controlled, and the casing, after full-tube heat treatment, exhibits even greater strength and toughness.

[0009] To achieve the above object, the present invention is implemented through the following technical solutions:

[0010] The present invention provides a method for manufacturing a high-strength and toughness casing, comprising the following steps: converter smelting, refining, continuous casting, slab reheating, controlled rolling, laminar cooling, coiling, welding, hot tension reducing and full-tube heat treatment; pre-annealing is performed before welding, and the pre-annealing step comprises: annealing the hot coil in a bell-type furnace, with the annealing cold spot temperature being 700-740°C, the holding time being 3.5-4.5h, and the furnace discharge temperature being ≤85°C.

[0011] As a further optimization solution of the present invention, in the coiling step: the coiling temperature is 550~600℃.

[0012] As a further optimization scheme of the present invention, the chemical composition of the casing and its weight percentage content are: C: 0.20~0.28%, Si: 0.2~0.3%, Mn: 0.8~1.0%, P: ≤0.010%, S: ≤0.003%, Cr: 0.1~0.3%, Ni: 0.05~0.15%, Cu: 0.10~0.30%, Als: 0.03~0.05%, Ti: 0.020~0.040%, Mo: ≤0.03%, B: ≤0.0030%, V: 0.045~0.070, and the balance is Fe and unavoidable inclusions.

[0013] As a further optimization solution of the present invention, the chemical composition of the casing must meet the following constraints:

[0014] .

[0015] Constraint Formula A d , which is used to limit the severity of the banded structure in the raw material coil before pre-annealing, and at the same time prevent the hardenability from being too strong, and forming martensite strips in the raw material coil before pre-annealing.

[0016] As a further optimization solution of the present invention, the raw material coil after pre-annealing: Rp 0.2 :340~370MPa,R m : 550~575MPa, yield strength ratio <0.65, elongation 30~35%, -10℃ longitudinal impact energy >90J, banded structure ≤level 2.

[0017] As a further optimization scheme of the present invention, the welding, hot stretch reducing and full tube heat treatment steps include: pre-annealed raw coils are welded into straight seam tube blanks by HFW, then heated to 920-1000°C for hot stretch reducing, and then heat treated by quenching at 890-920°C and tempering at 540-580°C to finally obtain the casing.

[0018] As a further optimization solution of the present invention, the casing after heat treatment: Rp 0.2 ≥900MPa, R m ≥985MPa, elongation ≥25%, longitudinal impact energy of parent material at -10℃ ≥130J.

[0019] As a further optimization scheme of the present invention, in the converter smelting and refining steps: the RH refining degassing time is ≥16min, the superheat of the tundish is controlled at 20~30°C, electromagnetic rollers and dynamic soft pressure are used during the continuous casting process, and the billet is placed in the insulation pit for slow cooling ≥72h.

[0020] As a further optimization scheme of the present invention, in the slab reheating, controlled rolling and laminar cooling steps: the billet out-of-furnace temperature is 1180~1220℃, the holding time is 120~160min, the finishing rolling temperature is 820~860℃, the rough rolling R1+R2 is fully descaling, the secondary descaling uses 30~40MPa ultra-high pressure, and the finishing rolling stand descaling is used.

[0021] As a further optimization solution of the present invention, the heated slab is rough rolled and finish rolled into a hot-rolled coil with a thickness of 10 to 16 mm.

[0022] As a further optimization solution of the present invention, in the low-temperature coiling step: the hot coil tower overflow during coiling should be ≤10mm.

[0023] The key chemical composition design ideas of the pre-annealed high-strength and toughness casing of the present invention are:

[0024] C: As the most critical element affecting strength and toughness in oil well casing steel, the reasons are as follows: (1) After pipe making, high-temperature tempering is used to obtain tempered bainite. The morphology and distribution of its carbides are key to achieving a good match between strength and toughness. If the C content is too low, the proportion of carbides in the tempered bainite is small, which especially affects the tensile strength; if the C content is too high, the plasticity and toughness after heat treatment will be significantly affected; (2) Increasing the C content is the key to improving the fatigue resistance and service life of continuous oil pipe steel. However, excessive C will form a more serious banded structure, which restricts weldability and HIC resistance. Taking all the above into consideration, the C content is 0.20~0.28%.

[0025] Si: As an element that expands the austenite zone, Si exists in the austenite in the form of a solid solution, promoting the formation of martensite and ferrite during the air cooling stage, thus expanding the cooling process window for the formation of soft ferrite during the air cooling stage. Furthermore, Si can significantly improve corrosion resistance. However, excessive Si content will significantly reduce the plasticity and toughness of the steel. Excessive Si content will also cause excessively thick reddish-brown oxide scale, which will also affect the surface quality of the steel plate. Therefore, the Si content should be controlled within a range of 0.2-0.3%.

[0026] Mn: It significantly improves strength through solid solution strengthening. Furthermore, higher Mn contents can reduce the critical cooling rate of steel, significantly improving hardenability and promoting the formation of martensite after in-line quenching. However, Mn has a large segregation coefficient during solidification, which can easily lead to segregation at grain boundaries and the formation of numerous MnS inclusions, resulting in poor low-temperature toughness. Furthermore, the resulting banded structure also exhibits significant Mn segregation, impairing HIC resistance. Therefore, a low Mn content of 0.8-1.0% is recommended.

[0027] Cr+Ni+Cu+B+Mo addition: (1) The casing needs to be heat treated throughout. The uniformity of the structure after quenching and tempering is closely related to the hardenability. Cr, Ni, and Mo are all elements that improve hardenability. This design uses Cr, Ni, and B to replace part of Mo, while taking into account economic efficiency and improving hardenability, forming a uniform quenched martensite. (2) The addition of Cr+Ni+Cu improves atmospheric corrosion resistance and HIC resistance. In summary, Cr: 0.1~0.3%; Ni: 0.05~0.15%; Cu: 0.10~0.30%; B: ≤0.0030%; Mo: ≤0.03%.

[0028] V and Ti: 1) Combines with C and N to form precipitates such as VC, VN, and TiC, increasing strength through grain refinement and precipitation strengthening. 2) Ti also removes some S by forming Ti4C2S2, reducing the S level in the steel, minimizing MnS formation, and promoting inclusion modification. 3) Some Ti can inhibit austenite grain coarsening in the coarse-grained zone during welding. Taking these factors into account, Ti levels are controlled between 0.020% and 0.040%, and V between 0.045% and 0.070%.

[0029] The above composition design and full-process process control of controlled rolling, controlled cooling, pre-annealing, and pipe making heat treatment are aimed at obtaining a high-strength and toughness casing that is easy to form and weld. In order to have good formability and weldability during the pipe making process, "medium C + low Mn + Cr + Ni + Cu + Ti + V" microalloying is adopted, and B and Mo are selectively added, combined with low-temperature coiling + hood annealing to obtain a low-strength raw material coil (yield strength 340~370MPa). The annealed coil is significantly softened compared to before pre-annealing, the yield strength is reduced by about 200MPa, the tensile strength is reduced by about 80~100MPa, and the yield strength ratio is reduced by about 0.15, which greatly reduces the difficulty of the roller forming operation. At the same time, the strip structure of the pre-annealing coil is significantly reduced (level 2) compared to the original hot coil, which improves the uniformity of the structure near the weld, reduces metal extrusion cracking during HFW welding, and improves HIC resistance. At the same time, after full pipe heat treatment, the pipe body further obtains better strength and toughness (Rp 0.2 ≥900MPa, R m ≥985MPa, elongation ≥25%. Longitudinal impact energy of parent material at -10℃ ≥130J).

[0030] The present invention effectively solves the technical difficulties in the manufacture of high-strength and high-toughness casing from multiple aspects such as component design and process control, and has produced significant beneficial effects in terms of raw material coil performance, finished pipe quality and production process:

[0031] The technical solution of this invention is mainly based on the "medium C + low Mn + Cr / Ni / Cu + Ti + V" composition design, with the selective addition of B and Mo. Through pre-annealing (700-740℃) + low-temperature coiling (550-600℃) + full tube heat treatment, the yield strength of the raw coil is 340-370MPa (a 40% reduction), the tensile strength is reduced to 550-575MPa, the yield strength ratio is less than 0.65, and the banded structure is ≤ Grade 2; the finished tube Rp 0.2≥900MPa, -10℃ impact energy ≥130J, HIC resistance without cracks. Existing technologies increase strength through high-temperature coiling or direct tempering, but fail to address the forming challenges caused by the high strength of the raw coil. The present invention reduces the yield strength of the raw coil to 340-370MPa through pre-annealing (700-740℃), significantly reducing the forming difficulty while maintaining a banded structure ≤Level 2 (compared to Level 4 without annealing). Coordinated composition design: "Cr / Ni / Cu" + optional addition of B partially replaces traditional high-Mo alloys, combined with Ti / V microalloying, to reduce costs while improving HIC resistance.

[0032] High strength and toughness: After pre-annealing, the elongation of the raw material coil reaches 30-35%, and the longitudinal impact energy at -10℃ is greater than 90J. It still has good impact resistance in low temperature environment and can meet the needs of complex working conditions. Good organizational uniformity: Pre-annealing makes the raw material coil banded structure ≤2 level. Compared with the unannealed raw material coil, it greatly reduces the risk of cracking near the weld and improves the organizational uniformity near the weld. After thermal tension reduction and full pipe heat treatment, the finished pipe Rp 0.2 ≥900MPa, R m ≥985MPa, elongation ≥25%, and longitudinal impact energy of the parent material at -10°C ≥130J, fully meeting the performance requirements of high-strength, high-toughness, large-diameter casing and suitable for unconventional oil and gas field production. Homogenized microstructure: Pre-annealed coils feature a homogenized F+P microstructure, while finished pipes feature a homogenized tempered bainite microstructure. Pre-annealing and tempering eliminate the adverse effects of banded microstructure on weldability and corrosion resistance, ensuring stable and reliable pipe performance. This ensures minimal performance fluctuations, excellent HIC resistance, and a long service life even under harsh conditions such as high pressure and corrosion. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the microstructure of the raw material roll after pre-annealing under an optical microscope according to Example 1 of the present invention. The microstructure after pre-annealing is F+P, and the banded microstructure is level 1.

[0034] Figure 2 This is the microstructure of the raw material coil without pre-annealing (Comparative Example 7), the microstructure is F+P, the banded structure is serious, and the assessment level is 4.

[0035] Figure 3 This is the microstructure of the finished tube of Example 1 of the present invention, which is homogenized tempered troostite. DETAILED DESCRIPTION

[0036] In order to enable those skilled in the art to better understand the technical solution of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, it should be understood that the drawings are only for illustrative purposes and cannot be understood as limiting this patent.

[0037] The present invention provides a method for manufacturing a high-strength and toughness casing. The chemical composition of the casing and the weight percentage content (wt%) thereof are as follows: C: 0.20-0.28%; Si: 0.2-0.3%; Mn: 0.8-1.0%; P: ≤0.010%; S: ≤0.003%; Cr: 0.1-0.3%; Ni: 0.05-0.15%; Cu: 0.10-0.30%; Als: 0.03-0.05%; Ti: 0.020-0.040%; B: ≤0.0030%; Mo: ≤0.03%; V: 0.045-0.070, and the balance is Fe and unavoidable inclusions.

[0038] The process includes: molten iron pretreatment → converter smelting → LF refining → RH refining → continuous casting → slab reheating → controlled rolling → laminar cooling → low-temperature coiling → pre-annealing → HFW welding, hot tension reducing, and full-tube heat treatment.

[0039] 1) Smelting, refining and continuous casting processes

[0040] Molten steel is smelted according to the above chemical composition. LF refining is followed by RH refining, with an RH degassing time of ≥16 minutes to ensure deep dehydrogenation and denitrification and more complete inclusion removal. The tundish superheat is controlled within a narrow range of 20-30°C to prevent nozzle blockage if too low, and coarse columnar grains in the slab, which leads to increased center segregation and, consequently, banded structure. To reduce center porosity and segregation, electromagnetic rollers and dynamic soft reduction are used during the continuous casting process, and the slabs are slowly cooled in a holding pit for ≥72 hours.

[0041] 2) Slab heating, controlled rolling and controlled cooling processes

[0042] The casting billet out of the furnace temperature is 1180~1220℃, and the holding time is 120~160min.

[0043] The heated slab undergoes rough rolling and finish rolling to produce 10-16mm hot-rolled coils. The finishing temperature for finish rolling is 820-860°C. To reduce the effects of surface decarburization and intergranular oxidation caused by subsequent raw coil annealing, low-temperature coiling is adopted, with a coiling temperature of 550-600°C.

[0044] Rough rolling R1+R2 is fully descaled, secondary descaling uses 30~40MPa ultra-high pressure, and descaling between finishing mill stands is used, which significantly improves the removal effect of dense oxide layer and reduces the peeling of iron oxide scale during hood annealing.

[0045] When coiling, the hot coil tower overflow should be ≤10mm to ensure smooth entry into the bell-type annealing furnace and prevent coil damage during vertical stacking.

[0046] 3) Pre-annealing

[0047] The hot coil is annealed in a bell-type furnace with an annealing cold point temperature of 700~740℃, a holding time of 3.5~4.5h, and a furnace outlet temperature ≤85℃.

[0048] 4) HFW welding, thermal tension reducing, full pipe heat treatment

[0049] After HFW welding into straight seam tube billets, medium frequency induction heating is used to heat the tube billets to 920~1000℃ and then hot tension reducing is performed; the tubes are then heat treated by quenching at 890~920℃ and tempering at 540~580℃.

[0050] The technical solution of the present invention is further illustrated below through specific examples and comparative examples.

[0051] The main chemical compositions of the examples and comparative examples are shown in Table 1, the main rolling process parameters are shown in Table 2, and the mechanical properties are shown in Table 3.

[0052] Table 1 Chemical compositions of the examples of the present invention and comparative examples

[0053]

[0054] Table 2 Main process parameters of the examples and comparative examples

[0055]

[0056] Table 3 Mechanical properties, impact properties and HIC resistance of examples and comparative examples

[0057]

[0058] from Figure 1 It can be seen that the structure after pre-annealing is F+P, and the banded structure is level 1. Combined with the data in Table 3, the banded structure of the pre-annealed raw material coil is ≤ level 2. According to the HIC test specified in NACE-2016 TM0284, no cracks were found. Figure 2 ) Banded structure is serious, the assessment level is 4, and the HIC test is also carried out, cracks appear. Therefore, it can be seen from the above that good structural uniformity improves the structural uniformity near the weld, reduces the risk of extrusion cracking, and also improves the comprehensive performance such as HIC resistance. Strength and plasticity balance: the strength of the raw steel coil after pre-annealing is reduced, Rp 0.2 340-370MPa, R m The tensile strength is 550-575MPa, the yield strength ratio is less than 0.65, and the elongation is 30-35%. As shown in Table 3, the Rp of the pre-annealed raw material coil in Example 1 is 0.2The Rp0.2 value is 341 MPa and the elongation is 35.0%. It maintains a certain strength while maintaining good plasticity. Compared with the unpre-annealed comparative example 7 (Rp0.2 is 570 MPa), it is easier to form, effectively reducing the difficulty of roll forming. High toughness: The longitudinal impact energy at -10°C is greater than 90J. For example, the raw material roll in Example 1 has an impact energy of 113 / 109 / 111J at -10°C. The excellent toughness meets the impact resistance requirements during subsequent processing and use.

[0059] After welding, thermal tension reduction and full pipe heat treatment, the casing strength and plastic toughness are further improved. 0.2 ≥900MPa, R m ≥985MPa, elongation ≥25%, parent material -10℃ longitudinal impact energy ≥130J. As in Example 1, the finished pipe Rp 0.2 903MPa, R m The tensile strength is 987MPa, the elongation is 26.5%, and the impact energy at -10℃ is 154 / 148 / 150J, which can meet the high requirements of casing strength and toughness for unconventional oil and gas field exploitation. Figure 3 The structure of the finished pipe is homogenized tempered troostite, which ensures the stability and reliability of the casing performance and enables it to withstand complex pressures and environments during use.

[0060] The present invention has been described in detail above with reference to the embodiments. However, the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof may be combined with each other unless there is a conflict.

Claims

1. A method for manufacturing high-strength and toughness casing, comprising the following steps: converter smelting, refining, continuous casting, slab reheating, controlled rolling, laminar cooling, coiling, welding, thermal tension reducing and full-tube heat treatment; characterized in that: Pre-annealing is performed before welding, and the pre-annealing step includes: annealing the hot coil in a bell-type furnace, with the annealing cold point temperature being 700-740°C, the holding time being 3.5-4.5h, and the furnace temperature being ≤85°C; In the controlled rolling step, the finishing temperature is 820-860°C; in the coiling step, the coiling temperature is 550-600°C; in the full tube heat treatment step, the quenching temperature is 890-920°C, and the tempering temperature is 540-580°C; The chemical composition of the sleeve and its weight percentage content are: C: 0.20-0.28%, Si: 0.2-0.3%, Mn: 0.8-1.0%, P: ≤0.010%, S: ≤0.003%, Cr: 0.1-0.3%, Ni: 0.05-0.15%, Cu: 0.10-0.30%, Als: 0.03-0.05%, Ti: 0.020-0.040%, Mo: ≤0.03%, B: ≤0.0030%, V: 0.045-0.070, and the balance is Fe and unavoidable inclusions.

2. The method for manufacturing a high-strength and toughness casing according to claim 1, characterized in that: The chemical composition of the casing must meet the following constraints:

3. The method for manufacturing a high-strength and toughness casing according to any one of claims 1 to 2, characterized in that: The raw material coil after pre-annealing: Rp 0.2 :340~370MPa,R m : 550~575MPa, yield strength ratio <0.65, elongation 30~35%, -10℃ longitudinal impact energy >90J, banded structure ≤level 2.

4. The method for manufacturing a high-strength and toughness casing according to claim 1, characterized in that: The welding and thermal tension reduction process comprises: welding the pre-annealed raw material coils into straight seam tube blanks by HFW, and then heating to 920-1000° C. for thermal tension reduction.

5. The method for manufacturing a high-strength and toughness casing according to claim 4, characterized in that: Heat treated casing: Rp 0.2 ≥900MPa, R m ≥985MPa, elongation ≥25%, longitudinal impact energy of base material at -10℃ ≥130J.

6. The method for manufacturing a high-strength and toughness casing according to claim 1, characterized in that: In the converter smelting and refining steps: the RH refining degassing time is ≥16 minutes, the superheat of the tundish is controlled at 20-30° C., electromagnetic rollers and dynamic soft pressure are used during the continuous casting process, and the cast billet is placed in a holding pit for slow cooling for ≥72 hours.

7. The method for manufacturing a high-strength and toughness casing according to claim 1, characterized in that: In the slab reheating, controlled rolling and laminar cooling steps: the billet out-of-furnace temperature is 1180-1220°C, the holding time is 120-160min, the rough rolling R1+R2 is fully descaled, the secondary descaling uses 30-40MPa ultra-high pressure, and the finishing rolling mill is used for descaling between stands.

8. The method for manufacturing a high-strength and toughness casing according to claim 7, characterized in that: The heated slab is rough rolled and finish rolled into a hot rolled coil with a thickness of 10 to 16 mm.

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