Ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel and production method thereof

Abstract

The invention relates to the technical field of steel production, and provides an ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel and a production method. The forged steel includes, by mass percentage, C≤0.06%, Si≤0.05%, 0.60%≤Mn≤1.00%, 6.0%≤Ni≤8.0%, 0.80%≤Cr≤1.20%, 1.0%≤Cu≤1.60%, 0.15%≤Al≤0.25%, Nb≤0.05%, P≤0.005%, S≤0.005%, gas elements [O]≤15ppm, [N]≤30ppm, [H]≤1ppm, and the remainder is Fe and unavoidable impurities. The invention realizes a significant improvement in the comprehensive performance of the forged steel through scientific component design, combined with advanced preparation technology, and precise control of the parameters of each step, and finally obtains a thick forged steel with high strength, high toughness and good corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to an ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel and a production method thereof. Background Art

[0002] With the rapid development of science and technology, the demand for high-performance materials in deep-sea exploration and petrochemical fields is increasing. In particular, the increasing diving depth of deep-sea equipment and the gradually increasing working pressure of petrochemical pressure vessels have put forward more stringent requirements on the performance of structural materials. Traditional materials can no longer meet the needs of these high-end applications. Therefore, the development of new high-performance materials has become one of the hot spots in the field of scientific research.

[0003] Deep-sea equipment, such as submersibles and submarine detectors, need to withstand extremely high hydrostatic pressure, which requires that their pressure-resistant structural materials not only have sufficient strength to resist external pressure, but also have excellent low-temperature toughness and corrosion resistance to cope with the extreme deep-sea environment. At the same time, as a key equipment for storing and transmitting high-pressure, high-temperature, and corrosive media, the manufacturing materials of petrochemical pressure vessels also need to achieve high strength, high toughness, and good welding performance to ensure the safe and stable operation of the equipment.

[0004] As the diving depth of deep-sea equipment increases and the pressure of pressure vessels increases, the strength of their structural materials gradually increases, and the application demand for 800MPa high-strength steel has been put forward. When using 800MPa-grade steel plates with a maximum thickness of 120mm for the pressure-resistant structure of deep-sea equipment and the manufacture of petrochemical pressure vessels, the existing 800MPa-grade high-strength steel plate materials have exposed some significant problems in practical applications:

[0005] (1) Difficulty in manufacturing: Especially when the thickness of the steel plate reaches more than 120 mm, the manufacturing process of "multiple steel plate forming + welding" becomes extremely complicated and difficult. The splicing of multiple steel plates not only increases the process steps, but also introduces more welded joints. These joints are often the weak links in the structure, which are prone to cause stress concentration and crack propagation.

[0006] (2) Forming and welding issues: The forming of thick steel plates requires large presses, which places extremely high demands on equipment capacity. At the same time, the welding of thick steel plates is also very difficult, and usually requires preheating before welding and heat treatment after welding to reduce welding stress and prevent cracks. These additional heat treatment steps not only increase manufacturing costs, but also extend the production cycle.

[0007] (3) High material performance requirements: The working environment of deep-sea equipment and petrochemical pressure vessels is complex and changeable, requiring the materials to have not only high strength, but also good toughness and weldability at low temperatures, as well as excellent corrosion resistance. However, traditional 800MPa grade steel plates often have difficulty meeting these requirements in terms of low-temperature toughness and corrosion resistance.

[0008] After consulting relevant patents and literature, it is found that the current research on 800MPa-grade high-strength materials is mainly concentrated in the field of steel plates, and most of these steel plates are produced by rolling forming. The 800MPa-grade steel plates / wires introduced in the invention patents CN103320690A, CN104294162A, CN110616369A, and CN111621711A are all rolled. Although rolling forming has high production efficiency, it has certain limitations in terms of material purity, organizational uniformity, and comprehensive performance control. Forged steel is forged by a hydraulic press / press, and the core production processes of the two are essentially different. In contrast, forged steel is forged by a hydraulic press or press, which can significantly improve the density and organizational uniformity of the material, thereby improving the comprehensive performance of the material.

[0009] However, existing high-strength forged steel products generally have a high carbon content in their composition design. Although high carbon content helps to improve the strength of the material, it also significantly reduces the welding performance and toughness of the material. According to literature such as "Influence of the Heat Affected Zone of 12CrNi5MoV Forged Steel Welding on Microstructure and Properties", "Study on the Influence of the Microstructure and Mechanical Properties of Ultra-thick 12CrNi5MoV Steel Forgings", and "Study on the Temper Brittleness Characteristics of 12CrNi5MoV Forged Steel", the carbon (C) content of 785MPa grade 12CrNi5MoV forged steel for ships is basically above 0.11%, and the carbon content and welding cold crack sensitivity of the material are high. The high-strength forged steel and large forged parts proposed by the invention patent CN108474071A disclosed in August 2018 have a carbon (C) content of 0.40% to 0.60% and a manganese (Mn) content of 0.7% to 1.2%. Combined with a reasonable forging process and heat treatment system, the forged steel has a tensile strength of not less than 850MPa, and an impact absorption energy of 22J to 109J at -20°C, which is mainly used for crankshaft manufacturing. The invention patent CN113462956A disclosed in October 2021 proposed a large-section, high-hardenability, high-strength medium-manganese forged steel and its preparation method. The forged steel has a carbon (C) content of 0.10% to 0.25%, a manganese (Mn) content of 5.0% to 9.9%, and a tensile strength of not less than 1000MPa, which is mainly used for engineering machinery manufacturing. It can be seen that in order to ensure the strength of forged steel, existing high-strength forged steel products all have a high carbon (C) content, but the welding performance is poor.

[0010] In view of this, the development of high-toughness and corrosion-resistant forged steel with a yield strength of 800MPa and large thickness has important practical significance. This new type of forged steel material can not only meet the requirements of deep-sea equipment and petrochemical pressure vessels for high strength, high toughness and corrosion resistance, but also reduce the welding workload and structural stress level through the overall forming process, thereby improving the construction efficiency and service performance of components.

[0011] In summary, the development of high-toughness and corrosion-resistant forged steel with a large thickness and a yield strength of 800 MPa is an effective way to meet the challenges of deep-sea equipment and petrochemical pressure vessel materials, and is of great significance for promoting technological progress and industrial development in related fields. Summary of the invention

[0012] In view of this, the present invention proposes an ultra-pure, large-thickness, 800MPa-grade high-toughness, corrosion-resistant forged steel and a production method, aiming to solve the demand for high-strength, high-toughness, and corrosion-resistant steels in the pressure-resistant structures of deep-sea equipment and petrochemical pressure vessels, as well as the problems encountered by traditional steels in the manufacturing process, such as difficult cold forming, large welding workload, and high structural stress.

[0013] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0014] The first object of the present invention is to disclose an ultra-pure, large-thickness, 800MPa-grade high-toughness, corrosion-resistant forged steel, whose chemical composition, in terms of mass percentage, includes: C≤0.06%, Si≤0.05%, 0.60%≤Mn≤1.00%, 6.0%≤Ni≤8.0%, 0.80%≤Cr≤1.20%, 1.0%≤Cu≤1.60%, 0.15%≤Al≤0.25%, Nb≤0.05%, P≤0.005%, S≤0.005%, gaseous elements [O] not higher than 15ppm, [N] not higher than 30ppm, [H] not higher than 1ppm, and the balance is Fe and unavoidable impurities.

[0015] The first object of the present invention is to disclose a production method of ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel, which is used to prepare the ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel as described above, comprising the following steps:

[0016] S1: Ultra-pure steel ingot manufacturing: adopt the process of electric arc furnace smelting + LF / VD refining + argon protection casting + argon protection electroslag to obtain ultra-pure steel ingots with target chemical composition;

[0017] S2: Forging and forming, the ultra-pure steel ingot prepared in step S1 is forged and formed through the processes of cutting, lengthening, roughening, forming and correcting;

[0018] S3: Performance heat treatment. After the steel ingot is forged, it needs to be heat treated to adjust its performance. Combined with the composition and performance requirements of the forged steel, the heat treatment process of the forged steel is "primary normalizing + secondary normalizing + two-phase zone quenching + tempering".

[0019] Furthermore, in step S1, a basic electric arc furnace is used to perform rough smelting of molten steel to obtain a chemical composition close to the target component. The electric arc furnace smelting adopts peroxidation operation, the argon pressure is controlled at 0.6-1.0 MPa, and Al blocks are added along with the steel flow when tapping.

[0020] Furthermore, in step S1, desulfurization and composition fine-tuning are performed through LF refining, and C powder, Al powder, diffusion deoxidizer, etc. are selected to deoxidize the molten steel.

[0021] Furthermore, in step S1, vacuum degassing is performed through VD refining to reduce the content of [O], [N], and [H] gases in the steel, the vacuum degree is not higher than 2 Torr, and the holding time is not less than 25 minutes.

[0022] Furthermore, in step S1, the consumable electrode is cast using vacuum, and the air in the casting system is replaced with argon at least 30 minutes before casting.

[0023] Furthermore, in step S1, a gas-shielded electroslag furnace is used to remelt the electrode rod, and the melting rate is controlled to be a constant melting rate of 20±3~30±3kg / min.

[0024] Furthermore, in step S2, the weight of the electroslag ingot head and tail cut before forging is not less than 7.5%, the forging steel is upset and drawn for not less than 2 times, the forging ratio is controlled above 9, and the forging temperature is set at 900°C to 1050°C.

[0025] Furthermore, in step S2, the forging billet is air-cooled to 500-550°C after forging, and then placed in a furnace for heat preservation for not less than 20 hours, and then furnace-cooled to room temperature.

[0026] Furthermore, in step S3, during the normalizing process, the forgings need to be completely austenitized and then kept warm. The holding time must ensure that the forgings can complete austenitization. The holding temperature of the normalizing process is set at 850-870°C, and the holding time is 3-5 min / mm. By setting the number of normalizing times for the forged steel twice, sufficient grain refinement is achieved; during the two-phase zone quenching process, the holding temperature is set at 740-780°C, and the holding time is 3-5 min / mm. Combined with water cooling, a two-phase zone quenching target structure of "martensite + a small amount of ferrite" is formed; during the tempering process, the holding temperature is 520-580°C, the holding time is 6-8 min / mm, and water cooling is used.

[0027] Compared with the prior art, the ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel and production method described in the present invention have the following advantages:

[0028] (1) The ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel described in the present invention can reach a thickness of 200mm, which can meet the requirements of deep-sea pressure resistance and petrochemicals; the yield strength exceeds 800MPa, the low-temperature impact absorption work is excellent, the corrosion resistance is good, and the corrosion rate is low; the forging process avoids the problems of cold forming and welding, improves the construction efficiency and service performance, and significantly improves the material performance.

[0029] (2) The production method of the ultra-pure large-thickness 800MPa-grade high-toughness corrosion-resistant forged steel described in the present invention optimizes the alloy composition and heat treatment process, and its purity is extremely high and the gas element content is extremely low, thereby ensuring the high quality of the steel. The strength, toughness and corrosion resistance of the steel all reach high levels, with a yield strength of not less than 800MPa, excellent low-temperature impact toughness, and corrosion resistance far exceeding that of traditional steel. Combined with the forging molding process adopted, welding workload is avoided, structural stress is reduced, and manufacturing efficiency and service performance of components are improved.

[0030] (3) The ultra-pure, thick, 800 MPa-grade high-toughness, corrosion-resistant forged steel and its production method described in the present invention achieve a significant improvement in the comprehensive performance of forged steel through scientific composition design, combined with advanced preparation technology, and precise control of parameters in each step. Ultimately, thick forged steel with high strength, high toughness and good corrosion resistance is produced, which significantly improves the comprehensive performance of the material and meets the demand for high-strength, high-toughness materials for deep-sea equipment pressure structures and petrochemical pressure vessels.

[0031] (4) The ultra-pure, thick, 800 MPa-grade high-toughness, corrosion-resistant forged steel and its production method described in the present invention provide a high-performance, easy-to-process steel solution, effectively promoting the technological progress of deep-sea equipment and petrochemical equipment.

[0032] (5) The ultra-pure, thick, 800 MPa-grade high-toughness, corrosion-resistant forged steel described in the present invention is suitable for the manufacture of submersible pressure-resistant structures and petrochemical pressure vessels. It can realize the integral forming of large components, avoid cold forming of high-strength steel, reduce welding workload, and reduce welding difficulty and structural stress level. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0034] Figure 1 This is a metallographic structure photograph of the transverse center of the forging plate of Example 1 of the present invention;

[0035] Figure 2 This is a photograph of the grain size structure at the transverse center of the forging plate of Example 1 of the present invention. DETAILED DESCRIPTION

[0036] In order to make the technical means of the present invention and the purpose and effect easy to understand, the embodiments of the present invention are described in detail below in conjunction with specific diagrams. In the description of this specification, the description of reference terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0037] In view of the higher performance requirements for materials put forward by deep-sea equipment pressure-resistant structures and petrochemical pressure vessels, the limitations of existing 800MPa-grade steel plates in terms of manufacturing difficulty, forming ability, welding performance and comprehensive performance are becoming increasingly prominent. The present invention proposes an innovative ultra-pure, large-thickness 800MPa-grade high-toughness corrosion-resistant forged steel and its production method. Through carefully designed low-carbon alloy components and strict preparation process control, the high strength, high toughness, corrosion resistance and large-thickness manufacturing capability of the forged steel are achieved, thereby meeting the stringent requirements for material performance of deep-sea equipment and petrochemical pressure vessels.

[0038] The invention discloses an ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel, whose chemical components, in terms of mass percentage, include: a carbon (C) content of not more than 0.06%, a silicon (Si) content of not more than 0.05%, a manganese (Mn) content of 0.60% to 1.00%, a nickel (Ni) content of 6.0% to 8.0%, a chromium (Cr) content of 0.80% to 1.20%, a copper (Cu) content of 1.0% to 1.60%, an aluminum (Al) content of 0.15% to 0.25%, a niobium (Nb) content of not more than 0.05%, phosphorus (P) and sulfur (S) contents of not more than 0.005%, gas elements [O] of not more than 15ppm, [N] of not more than 30ppm, [H] of not more than 1ppm, and the remainder is iron (Fe) and unavoidable impurities.

[0039] The ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel described in the present invention, firstly, adopts low-carbon (C) design for cast steel to reduce carbon equivalent and improve weldability of forged steel; at the same time, the reasonable addition of 6.0% to 8.0% nickel (Ni), 0.80% to 1.20% chromium (Cr), and 0.8% to 1.2% copper (Cu) plays a strengthening role on the one hand to ensure higher strength, and at the same time can improve casting fluidity and corrosion resistance; in addition, the ultra-low content control of alloy element Si, impurity elements P, S, and gas elements [O], [N], [H] is conducive to ensuring the low-temperature toughness of steel; a certain amount of alloy element Al, combined with Ni, is used to make up for the strength loss caused by low carbon. Through this series of scientific component designs, combined with advanced preparation technology, a thick forged steel with high strength, high toughness and good corrosion resistance is finally obtained, which significantly improves the comprehensive performance of the material.

[0040] The ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel of the present invention has a thickness of up to 200mm, which can meet the needs of deep-sea pressure resistance and petrochemicals; the yield strength exceeds 800MPa, the low-temperature impact absorption work is excellent, the corrosion resistance is good, and the corrosion rate is low; the forging process avoids the problems of cold forming and welding, improves construction efficiency and service performance, and significantly improves material performance.

[0041] The present invention also discloses a method for producing ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel, comprising the following steps:

[0042] S1: Ultra-pure steel ingot manufacturing: adopt the process of electric arc furnace smelting + LF / VD refining + argon protection casting + argon protection electroslag to obtain ultra-pure steel ingots with target chemical composition;

[0043] S2: Forging and forming, the ultra-pure steel ingot prepared in step S1 is forged and formed through the processes of cutting, lengthening, roughening, forming and correcting;

[0044] S3: Performance heat treatment. After the steel ingot is forged, it needs to be heat treated to adjust its performance. Combined with the composition and performance requirements of the forged steel, the heat treatment process of the forged steel is "primary normalizing + secondary normalizing + two-phase zone quenching + tempering".

[0045] The production method of the ultra-pure, large-thickness, 800MPa-grade high-toughness, corrosion-resistant forged steel described in the present invention ensures that the ingot has good purity through the manufacture of ultra-pure steel ingots, laying a technical foundation for subsequent performance control. First, by accurately controlling the content of alloying elements such as carbon, silicon, manganese, nickel, chromium, copper, aluminum, etc., the high strength of the steel is guaranteed, and its low-temperature toughness and corrosion resistance are improved. At the same time, the content of phosphorus, sulfur and gas elements such as oxygen, nitrogen, and hydrogen is strictly controlled to ensure the purity of the steel. In the preparation process, advanced processes such as arc furnace smelting, LF / VD refining, argon protection casting and electroslag remelting are used to further purify the molten steel and reduce the content of inclusions and gas. Subsequently, the density and uniformity of the steel are improved through a multi-step forging process, and then through a unique heat treatment system (one normalizing + secondary normalizing + two-phase zone quenching + tempering), grain refinement is achieved, so that the strength and toughness of the steel are optimally matched. Specifically, the production method of ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel described in this application, wherein "arc furnace smelting + LF / VD refining + argon protection pouring + argon protection electroslag", each process is well coordinated to achieve the ultra-pure component control requirements, "arc furnace smelting" realizes the original purity control, "LF / VD refining" effectively controls the impurity elements and gas elements in the steel, "argon protection pouring" prevents the entry of atmospheric gas, "argon protection electroslag" further reduces the inclusion content, and prevents gas from entering. Forging can effectively improve the density and uniformity of steel, and effectively guarantee the uniformity of the performance of thick forged steel. The requirements of each process complement each other, "ingot cutting" helps to improve the purity of forgings and avoid internal defects, "upsetting times and forging ratio" are important guarantees for improving uniformity, and "forging temperature" and "post-forging heat treatment system" are important guarantees for good quality of forgings. The performance hot-out process parameters directly determine whether the performance of the steel plate meets the requirements. The combination of "one normalizing + two normalizing + two-phase zone quenching + tempering" can achieve grain refinement and good matching of strength and toughness. All performances meet the index requirements. "Normalizing" is to refine the grain size of forged steel ingots, and "two-phase zone quenching" adjusts the ingot structure to the ideal structure of "martensite + a small amount of ferrite". The value range of a small amount of ferrite is 10%~20%. "Tempering" restores martensite, and carbides, copper precipitates, and NiAl precipitates evenly and finely, achieving a good match of strength and toughness of forged steel.

[0046] In the forging steel composition of the present invention, the reasons for limiting the main alloying elements are described as follows:

[0047] Carbon (C) is the most important solid solution strengthening element, which can significantly improve the strength of the steel plate, but is not good for the impact toughness of the steel, especially the upper platform impact energy, and also significantly reduces the welding performance of the steel. Taking all factors into consideration, the carbon content range of the present invention is limited to no more than 0.06%.

[0048] Silicon (Si) is one of the commonly used deoxidizing elements and also has a certain strengthening effect, but silicon will reduce the cleanliness of steel and deteriorate its toughness and weldability. Taking comprehensive considerations, the present invention limits the silicon content range to no more than 0.05%.

[0049] Manganese (Mn) is an effective element for improving strength and hardenability, but manganese has a high tendency to segregate, so its content should not be too high. In order to ensure the hardenability of steel and avoid serious segregation, the present invention limits the manganese content to 0.6% to 1.0%.

[0050] Nickel (Ni) can improve the strength of steel, prevent hot cracking during heating and forging, and improve the low-temperature toughness and corrosion resistance of steel. However, too high a content of nickel is prone to composition deviation and high production cost. The present invention limits the nickel content range to 6.0% to 8.0%.

[0051] Chromium (Cr) can play a significant role in solid solution strengthening and can significantly improve the hardenability and corrosion resistance of steel, but a high chromium content will reduce the plasticity and weldability of the steel. In the present invention, the chromium content range is limited to 0.80% to 1.20%.

[0052] Copper (Cu) has solid solution strengthening and precipitation dispersion strengthening effects, which can improve yield strength, tensile strength and corrosion resistance. However, excessive copper content can easily cause surface cracks. In the present invention, the copper content range is limited to 1.0-1.6%.

[0053] Aluminum (Al) combines with nickel to form NiAl precipitates, which can play a precipitation strengthening role, but due to the low density of aluminum, it is easy to float in the molten pool during welding, thereby significantly reducing the weldability of steel. In the present invention, the aluminum content range is limited to 0.15% to 0.25%.

[0054] Niobium (Nb) can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, increase the austenite recrystallization temperature, refine the grains, and improve the toughness of steel during the forging process; the precipitated carbonitride particles have a precipitation strengthening effect, but the solubility of niobium in steel is limited, and too high a content will deteriorate the plasticity and toughness. In the present invention, the niobium content range is limited to no more than 0.05%.

[0055] Phosphorus (P) and sulfur (S) are common impurity elements in steel. Phosphorus tends to segregate at grain boundaries, reducing the toughness of steel; sulfur tends to form inclusions with other elements, reducing the strength-toughness match. In the present invention, the phosphorus and sulfur contents are limited to no more than 0.005%.

[0056] Oxygen [O], nitrogen [N], and hydrogen [H] are gaseous elements that enter during the steelmaking process. Oxygen [O] easily forms oxides (inclusions) with alloys, and nitrogen [N] easily forms precipitates with niobium and carbon, thus affecting the low-temperature toughness of steel. [H] will aggregate during the preparation of high-strength steel, affecting the plasticity and toughness of steel, and even causing hydrogen embrittlement cracks that lead to material scrapping. Taking all factors into consideration, the gaseous element [O] should not be higher than 15ppm, [N] should not be higher than 30ppm, and [H] should not be higher than 1ppm.

[0057] The production method of the ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel described in the present invention optimizes the alloy composition and heat treatment process, and its purity is extremely high, and the gas element content is extremely low, ensuring the high quality of the steel. The strength, toughness and corrosion resistance of the steel have reached a high level, the yield strength is not less than 800MPa, the low-temperature impact toughness is excellent, and the corrosion resistance is far superior to that of traditional steel. Combined with the forging molding process adopted, the welding workload is avoided, the structural stress is reduced, and the manufacturing efficiency and service performance of the components are improved. These characteristics make the forged steel of the present invention particularly suitable for the manufacture of deep-sea equipment pressure structures and petrochemical pressure vessels, and can significantly improve the overall performance and reliability of these equipment.

[0058] As a preferred example of the present application, in step S1, an alkaline arc furnace is used to perform rough smelting of molten steel to obtain a chemical composition close to the target component. The arc furnace smelting adopts peroxidation operation, the argon pressure is controlled at 0.6-1.0 MPa, and Al blocks are added along with the steel flow when tapping.

[0059] In this process step, a basic arc furnace is used to roughly refine molten steel. Through peroxidation operation and precise control of argon pressure in the range of 0.6-1.0MPa, the surrounding air is effectively isolated, and the content of nitrogen, hydrogen and oxygen in the molten steel is significantly reduced. At the same time, when tapping, aluminum blocks are added in time with the steel flow to prevent serious burning that may be caused by early addition. This step can greatly improve the initial purity of molten steel and lay a solid foundation for the subsequent refining process.

[0060] As a preferred example of the present application, in step S1, desulfurization and composition fine-tuning are performed through LF refining, and C powder, Al powder, diffusion deoxidizer, etc. are selected to deoxidize the molten steel. In step S1 of ultra-pure steel ingot manufacturing, refining treatment is performed through LF (ladle refining furnace), and desulfurization operation is first realized to effectively reduce the sulfur content in the molten steel, reduce the adverse effects of sulfur on the performance of steel, and improve the purity and quality of the steel; then, the composition is fine-tuned to ensure that the composition of the molten steel is close to the target design value, laying the foundation for the production of high-quality forged steel. In the deoxidation process, C powder and Al powder are selected as diffusion deoxidizers, which are evenly sprinkled into the molten steel, react with oxygen in the molten steel, generate gas to escape, and thus reduce the oxygen content in the molten steel. In this process, the use of traditional deoxidizers such as SiAlBaCa is particularly avoided to prevent the purity of the molten steel and the performance of the final product from being affected by excessive addition of silicon.

[0061] As a preferred example of the present application, in step S1, vacuum degassing is performed by VD refining to reduce the content of [O], [N], and [H] gases in the steel, with a vacuum degree of no more than 2 Torr and a holding time of no less than 25 minutes. In the manufacturing process of ultra-pure steel ingots, VD refining degasses the molten steel in a high vacuum environment, and uses a vacuum pump to extract gas components such as oxygen ([O]), nitrogen ([N]), and hydrogen ([H]) in the molten steel, thereby significantly reducing the content of these harmful gases. During the VD refining process, the vacuum degree is precisely controlled at a level not higher than 2 Torr, and the vacuum state is maintained for no less than 25 minutes to ensure that the gas components in the molten steel are fully removed. This long-term, high-vacuum treatment provides a guarantee for high-quality degassing of the molten steel.

[0062] The purity of molten steel after VD refining treatment has been significantly improved, and it shows more excellent performance in strength, toughness, corrosion resistance, etc., fully meeting the production requirements of ultra-pure, large-thickness 800MPa-grade high-toughness and corrosion-resistant forged steel.

[0063] As a preferred example of the present application, in step S1, vacuum is used for consumable electrode casting, and argon is used to replace the air in the casting system at least 30 minutes before casting. In the manufacturing process of ultra-pure steel ingots, the step of using vacuum for consumable electrode casting is to ensure that the molten steel is not exposed to the air during the entire casting process, thereby preventing impurities such as oxygen and nitrogen in the air from entering the molten steel. At least 30 minutes before casting, preferably one hour before casting, argon is used to replace the air in the casting system to ensure that the inside of the casting system is filled with an inert gas environment, which can minimize the direct contact between the molten steel and the air. Argon is selected as the replacement gas because of its stable chemical properties and does not react with molten steel. Through this step, the molten steel is always kept in a low-oxygen, low-nitrogen environment during the casting process, thereby ensuring the purity of the steel ingot.

[0064] As a preferred example of the present application, in step S1, a gas-protected electroslag furnace is used to remelt the electrode rod, and its melting rate is controlled to be a constant melting rate of (20-30) ± 3 kg / min. The purity of the steel ingot is improved by using a gas-protected electroslag furnace to remelt the electrode rod. During this process, the electrode rod is placed in the gas-protected electroslag furnace and remelted by the resistance heat generated by the current, and the melting rate is controlled within a constant range ((20-30) ± 3 kg / min), avoiding the problems that may be caused by too fast or too slow melting rates, such as internal defects or stress concentration, and ensuring the uniformity and integrity of the internal structure of the steel ingot. At the same time, the argon protection is continuously turned on to continuously replace the air in the furnace, maintaining the oxygen content at an extremely low level (not higher than 0.5%), preventing the molten steel from being oxidized during the remelting process, and reducing the mixing of other harmful gases such as nitrogen and hydrogen, ensuring that the steel ingot can maintain extremely high purity during the remelting stage, while reducing the possibility of component segregation.

[0065] This series of operations is crucial for the production of ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel. These measures work together to ensure that the final steel ingot has higher purity and fewer internal defects, providing a better raw material basis for subsequent heat treatment and forging, and ensuring the excellent performance of the final product, the ultra-pure steel ingot. The high-purity and low-segregation steel ingot is easier to obtain the ideal organization and performance in the subsequent forging and heat treatment process, meeting the strict requirements for material performance for high-end applications such as deep-sea equipment pressure-resistant structures and petrochemical pressure vessels.

[0066] As a preferred example of the present application, in step S2, the weight of the head and tail cut of the electroslag ingot before forging is not less than 7.5%, the forging steel is upset and drawn for not less than 2 times, the forging ratio is controlled above 9, and the forging temperature is set at 900°C to 1050°C. By implementing the above-mentioned forging forming workflow, first, the electroslag ingot is cut at the ends before forging, and the weight of the cut-off is not less than 7.5%. This step is aimed at removing impurities and areas with more internal defects at both ends of the ingot, which significantly improves the purity and overall quality of the forging; then, through multiple upsetting and drawing and large forging ratio (controlled at above 9) treatment, the ingot undergoes plastic deformation under the action of triaxial stress, thereby improving or eliminating the component segregation inside the ingot, improving the internal quality, and reducing the anisotropy of the material, making the internal composition of the ingot more uniform; finally, the forging temperature is set between 900℃ and 1050℃ to ensure that the ingot can be fully plastically deformed within this temperature range, and will not cause coarse grains, overburning or decarburization due to excessively high temperature, nor will it increase the forging difficulty or even cause cracking due to excessively low temperature, ensuring that the organizational structure and mechanical properties of the forging meet the expected goals. During the forging process, the steel ingot undergoes the above-mentioned series of precise operating steps to achieve the desired shape and performance, so that the final forgings not only have excellent physical and chemical properties, but also meet the strict requirements of high-end application fields such as deep-sea equipment pressure-resistant structures and petrochemical pressure vessels.

[0067] As a preferred example of the present application, in step S2, after forging, the forging billet is air-cooled to (500-550)°C and then put into a furnace for heat preservation for not less than 20 hours and then cooled to room temperature in the furnace.

[0068] This step further defines the cooling method of the steel ingot after forging. After the forging blank is air-cooled to a specific temperature range (500-550°C), it is sent to a heating furnace for a long period of insulation treatment (not less than 20 hours), and then naturally cooled to room temperature. The air cooling process helps to quickly reduce the surface temperature of the forging blank. The subsequent insulation in the furnace provides a continuous and stable heat environment, so that the hydrogen atoms inside the forging blank have enough time to diffuse to the surface of the forging blank and escape, thereby reducing the hydrogen content in the steel ingot; at the same time, long-term insulation can also effectively release the residual stress inside the forging blank, prevent cracks caused by stress concentration in subsequent processing, and improve the processing performance and service life of the forging.

[0069] This design process effectively reduces the hydrogen content in the forging blank, reduces the risk of hydrogen-induced cracks, avoids crack problems caused by stress concentration during subsequent processing or use, and helps to homogenize and stabilize the internal structure of the forging blank, providing a better material basis for subsequent mechanical processing and heat treatment.

[0070] As a preferred example of the present application, in step S3, during the normalizing process, the forgings need to be completely austenitized and then kept warm. The holding time must ensure that the forgings can complete austenitization. The holding temperature of the normalizing process is set to (850-870)°C, and the holding time is (3-5) min / mm. By setting the forged steel normalizing times twice, sufficient grain refinement is achieved; during the two-phase zone quenching process, the holding temperature is set to (740-780)°C, and the holding time is (3-5) min / mm. Combined with water cooling, a forged steel two-phase zone quenching target structure of "martensite + (10-20%) ferrite" is formed; during the tempering process, the holding temperature is set to (520-580)°C, the holding time is (6-8) min / mm, and water cooling is used.

[0071] The above settings optimize and adjust the internal structure of forged steel by precisely controlling the heating temperature and holding time. First, the normalizing treatment keeps the forgings at a specific temperature, which promotes the austenite transformation of the internal structure of the material, and refines the grains through two normalizings to improve the uniformity of the material. Then, the two-phase zone quenching treatment allows the material to cool rapidly to form a high-hardness martensite structure, while retaining a small amount (10-20%) of ferrite to ensure toughness. Finally, the tempering treatment adjusts the strength and toughness of the material to the optimal balance by controlling the lower heating temperature and time so that the carbides in the martensite precipitate and are evenly distributed. Throughout the process, the parameter settings of each step are based on the scientific analysis of the composition characteristics of the steel and the heat treatment effect, aiming to achieve the predetermined performance goals. By implementing the performance heat treatment process of "one normalizing + two normalizing + two-phase zone quenching + tempering", combined with the precise control of the parameters of each step, the comprehensive performance of forged steel has been significantly improved, meeting the needs of high-strength and high-toughness materials for deep-sea equipment pressure-resistant structures and petrochemical pressure vessels.

[0072] By adopting the above-mentioned production method of ultra-pure, thick, 800MPa grade high-toughness, corrosion-resistant forged steel of the present invention, the following effects can be achieved:

[0073] (1) The purity of the forged steel of the present invention is relatively high: [O]≤15ppm, [N]≤30ppm, [H]≤1ppm, and the maximum thickness can reach 200mm.

[0074] (2) The forged steel of the present invention has a high level of toughness: the yield strength is not less than 800 MPa, and the average value of the Charpy impact energy absorption at -80°C is not less than 170 J.

[0075] (3) The forged steel of the present invention has good corrosion resistance, and the annual average corrosion rate is not higher than 0.8 mm / a.

[0076] (4) The forged steel of the present invention can be used for forming large structural parts, and is expected to be used in the manufacture of pressure-resistant structures of ship deep-sea equipment and petrochemical pressure vessels.

[0077] In order to clearly illustrate the technical features of this solution, the present invention is further described below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments.

[0078] Examples 1 to 3 adopt the "arc furnace smelting + LF / VD refining + argon protection pouring + argon protection electroslag" process to prepare forging blanks, and each element of the forging blank meets the design requirements; the contents of impurity elements P, S, [O], [N], and [H] in the forgings are all low, proving that the smelting process is reasonable and feasible.

[0079] Example 4 adopts "arc furnace smelting + LF / VD refining + atmospheric casting" to prepare forging blanks. The contents of impurities and gas elements P, S, [O], [N], and [H] in the forging blanks are significantly higher than those in Examples 1 to 3, and do not meet the composition design requirements.

[0080] Example 5 adopts "arc furnace smelting + LF / VD refining + argon protection casting + ordinary electroslag" to prepare forging blanks. The contents of impurities and gas elements P, S, [O], [N], and [H] in the forging blanks are all lower than those in Example 4 to a certain extent, but the gas elements [O], [N], and [H] are significantly higher than those in Examples 1 to 3, and the [H] content does not meet the composition design requirements.

[0081] Table 1 Chemical composition of forged steel of the present invention (wt%)

[0082]

[0083] Forging blanks with the components of Examples 1 to 3 were forged, and the specific process parameters are shown in Table 2. Examples 6 and 7 in the table have the same components as Example 1, but different forging processes, which are mainly reflected in the smaller forging ratio.

[0084] Table 2 Forging system of forged steel of the present invention

[0085]

[0086] The forging blanks of Examples 1 to 3, 6 and 7 were subjected to performance heat treatment, and the heat treatment process parameters are shown in Table 3. In the table, Examples 8 and 9 have the same composition and forging process as Example 2, but different heat treatment process parameters, mainly reflected in the different normalizing times.

[0087] Table 3 Wall thickness and heat treatment process of forged steel of the present invention

[0088]

[0089] The mechanical property test results of the forgings of Examples 1, 2, 3, 6, 7, 8 and 9 of the present invention are shown in Table 4. (1) The forgings of Examples 1 to 3 have good strength and toughness matching, relatively fine grain size / small longitudinal and transverse difference, yield strength ≥ 800 MPa, -80°C impact absorption energy ≥ 170 J, and grain size grade ≥ 8.0, which shows that their composition and process are reasonable and feasible; (2) Compared with Example 1, Example 6 has large longitudinal and transverse differences in tensile properties, low -80°C impact absorption energy, and relatively coarse grains. Example 7 also shows a certain directionality, so it is very necessary to control the forging ratio to not less than 9; (3) Compared with Example 2, Example 8 has slightly lower -80°C impact absorption energy and slightly coarse grain size. Example 9 is basically equivalent to its performance, so it is reasonable and feasible to set the number of normalizing times to 2.

[0090] Table 4 Mechanical properties test results of forged steel of the present invention

[0091]

[0092] The forgings of Examples 1, 2, and 3 were sampled and subjected to indoor accelerated corrosion tests under laboratory conditions. For comparison purposes, the corrosion rate of 800MPa grade 12CrNi5MoV forged steel was measured simultaneously, and the corrosion test medium was 3.5% NaCl solution. After the samples were subjected to 30 days of corrosion test, the average corrosion rate measured is shown in Table 5. It can be seen that the forged steel has good corrosion resistance, and the annual average corrosion rate is not higher than 0.80mm / a, which is lower than that of 12CrNi5MoV forged steel.

[0093] Table 5 Average corrosion rate of materials after 30 days of full immersion corrosion

[0094]

[0095] Figure 1 , Figure 2 This is a photo of the metallographic structure and grain size structure of the transverse center position of the plate of Example 1 of the present invention. The structure is tempered troostite + 16% ferrite, indicating that the forged steel has good hardenability; the grain size is 8.0 level, and the size is relatively uniform and small.

[0096] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for producing ultra-pure, thick, 800MPa grade high-toughness, corrosion-resistant forged steel, characterized in that: The ultra-pure, thick, 800MPa-grade high-toughness, corrosion-resistant forged steel has the following chemical compositions by mass percentage: C≤0.06%, Si≤0.05%, 0.60%≤Mn≤1.00%, 6.0%≤Ni≤8.0%, 0.80%≤Cr≤1.20%, 1.0%≤Cu≤1.60%, 0.15%≤Al≤0.25%, Nb≤0.05%, P≤0.005%, S≤0.005%, gaseous elements [O] not higher than 15ppm, [N] not higher than 30ppm, [H] not higher than 1ppm, and the remainder is Fe and unavoidable impurities. The production method thereof comprises the following steps: S1: Ultra-pure steel ingot manufacturing: adopt the process of electric arc furnace smelting + LF / VD refining + argon protection casting + argon protection electroslag to obtain ultra-pure steel ingots with target chemical composition; Wherein, in step S1, a basic electric arc furnace is used to perform rough smelting of molten steel to obtain a chemical composition close to the target component, the electric arc furnace smelting adopts peroxidation operation, the argon pressure is controlled at 0.6-1.0 MPa, and Al blocks are added with the steel flow during steel tapping; In step S1, the consumable electrode is cast using vacuum, and the air in the casting system is replaced with argon at least 30 minutes before casting; In step S1, a gas-protected electroslag furnace is used to remelt the electrode rod, and its melting rate is controlled to be a constant melting rate of 20-30 kg / min; S2: Forging and forming, the ultra-pure steel ingot prepared in step S1 is forged and formed through the processes of cutting, lengthening, roughening, forming and correcting; Wherein, in step S2, the weight of the electroslag ingot before forging is not less than 7.5%, the forging steel is upset and drawn for not less than 2 times, the forging ratio is controlled to be above 9, the forging temperature is set to 900℃~1050℃, and the forging blank is air-cooled to 500~550℃ after forging, and then put into the furnace for heat preservation for not less than 20h, and then cooled to room temperature in the furnace; S3: Performance heat treatment. After the steel ingot is forged, it needs to be heat treated for performance control. Combined with the composition and performance requirements of forged steel, the heat treatment process of forged steel is "one normalizing + two normalizing + two-phase quenching + tempering". In the normalizing process, the forgings need to be completely austenitized and kept warm. The holding time must ensure that the forgings can complete austenitization. The holding temperature of the normalizing process is set at 850-870℃, and the holding time is 3-5min / mm. By setting the number of normalizing times for forged steel twice, the grains are fully refined. In the two-phase quenching process, the holding temperature is set at 740-780℃, and the holding time is 3-5min / mm. Combined with water cooling, the target organization of the two-phase quenching state of forged steel of "martensite + a small amount of ferrite" is formed. The value range of a small amount of ferrite is 10%-20%; the tempering process adopts a holding temperature of 520-580℃, a holding time of 6-8min / mm, and water cooling.

2. The method for producing ultra-pure, thick, 800MPa grade high-toughness, corrosion-resistant forged steel according to claim 1, characterized in that: In step S1, desulfurization and composition fine-tuning are carried out through LF refining, and C powder, Al powder and diffusion deoxidizer are selected to deoxidize the molten steel.

3. The method for producing ultra-pure, thick, 800MPa grade high-toughness, corrosion-resistant forged steel according to claim 1, characterized in that: In step S1, vacuum degassing is performed through VD refining to reduce the gas contents of [O], [N], and [H] in the steel. The vacuum degree is not higher than 2 Torr and the holding time is not less than 25 minutes.

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