An ultra-high strength and large thickness forged steel and its preparation method

Abstract

The present invention provides an ultra-high strength and large-thickness forged steel and a preparation method thereof. The chemical composition of the forged steel by weight percentage includes: C 0.08% - 0.18%, Mn 0.80% - 1.50%, Ni 7.0% - 10.0%, Cr 0.60% - 1.60%, Mo 0.50% - 1.20%, Si ≤ 0.40%, V ≤ 0.15%, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and inevitable impurities. The forged steel obtained by using the preparation method of the present invention has a yield strength of 1016 MPa - 1086 MPa, a tensile strength of 1107 MPa - 1223 MPa, an elongation after fracture of 16.0% - 18.0%, and an impact energy absorption at -20°C of 113 J - 193 J. This ultra-high strength and large-thickness forged steel can be applied to the construction of deep-sea assembly pressure-resistant structures.

Description

Technical Field

[0001] The present invention relates to the technical field of deep - sea exploration equipment, and more particularly, to an ultra - high strength and large - thickness forged steel and a preparation method thereof. Background Art

[0002] Forged steel is an indispensable material for the construction of pressure vessel steel structures. It is used for the overall manufacturing of ring - shaped, cylindrical, conical, and complex cross - section structural parts, which can form a streamlined structure, reduce the welding workload and welding deformation, and lower the manufacturing difficulty. With the development of China's ocean exploration technology, the diving depth of deep - sea equipment has been continuously increasing, and there is an urgent need for ultra - high strength forged steel with a yield strength of not less than 1000 MPa, a maximum thickness of not less than 260 mm, and an impact requirement of not less than 100 J at - 20°C for the construction of its pressure - resistant structure.

[0003] To ensure the safe, reliable and normal service of deep-sea equipment and the like, the properties of large-thickness forged steel materials used in their construction must have characteristics such as high strength and good low-temperature toughness. After consulting relevant patents and literature, for the ultra-high-strength steel plates with a thickness of no more than 100 mm for armored vehicles specified in GJB 31A, such as 603, 617, 675, etc., and the bulletproof steel plates for ships with a thickness of no more than 20 mm specified in GJB 3935, and the ultra-high-strength steels introduced in invention patents CN101624681A, CN104674121A, CN109628835A, CN111394652A, etc., their preparation all requires rolling through a rolling mill, which is essentially different from forging. The invention patent CN112626412A published in November 2020 proposed a corrosion-resistant high-strength low alloy and its preparation method. The carbon (C) content of this forged steel is 0.33% - 0.38%, the chromium (Cr) content is 0.95% - 1.20%, the nickel (Ni) content is 0.16% - 0.25%, and the molybdenum (Mo) content is 0.21% - 0.30%. This steel has good low-temperature toughness, and the impact energy absorption at -20°C is not less than 100 J. It is mainly used for the manufacture of deep-sea oil extraction equipment, but the yield strength of this steel is 840 MPa - 930 MPa. The invention patent CN113462956A published in May 2021 proposed a large-section high-hardening high-strength medium-manganese forged steel and its preparation method. The carbon (C) content of this forged steel is 0.10% - 0.25%, the silicon (Si) content is 0.1% - 0.2%, and the manganese (Mn) content is 5.0% - 9.9%. The tensile strength is not less than 1000 MPa, and the hardened layer depth is not less than 200 mm. This forged steel has good hardenability and relatively high hardness, but its low-temperature toughness is unknown. The invention patent CN116043114A published in January 2023 proposed a high-toughness alloy forged steel for high-speed rail brake discs with a speed of 400 km / h, its heat treatment method and production method. The carbon (C) content of this forged steel is 0.18% - 0.25%, the silicon (Si) content is 0.15% - 0.40%, the manganese (Mn) content is 0.4% - 0.8%, the chromium (Cr) content is 0.50% - 1.20%; the nickel (Ni) content is 0.50% - 0.70%, and the molybdenum (Mo) content is 0.3% - 0.60%. The yield strength is not less than 1100 MPa, and the impact energy absorption at 20°C is not less than 150 J, but its thickness is only 80 mm. It is not difficult to see that the existing inventions mainly focus on the research of ultra-high-strength steel plates. The limited strength, impact toughness and thickness of high-strength forged steel are difficult to simultaneously meet the construction requirements of the pressure-resistant structure of ultra-high-strength forged steel with a yield strength of not less than 1000 MPa, a maximum thickness of not less than 260 mm, and an impact requirement of not less than 100 J at -20°C. Therefore, it is necessary to carry out the research and development of ultra-high-strength and high-toughness forged steel. Summary of the Invention

[0004] In view of this, the present invention aims to provide an ultra-high strength and large-thickness forged steel and a preparation method thereof, so as to solve the problem that the existing technology mainly focuses on the research of ultra-high strength steel plates, and the limited strength, impact toughness and thickness of high-strength forged steels are difficult to simultaneously meet the demand for the construction of pressure-resistant structures of ultra-high strength forged steels with a yield strength of not less than 1000 MPa, a maximum thickness of not less than 260 mm, and an impact requirement of not less than 100 J at -20°C. Therefore, it is necessary to develop ultra-high strength and high-toughness forged steels.

[0005] To achieve the above object, the technical solution of the present invention is realized as follows:

[0006] An ultra-high strength and large-thickness forged steel, the chemical composition of the forged steel by weight percentage includes: C 0.08% - 0.18%, Mn 0.80% - 1.50%, Ni 7.0% - 10.0%, Cr 0.60% - 1.60%, Mo 0.50% - 1.20%, Si ≤ 0.40%, V ≤ 0.15%, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and inevitable impurities.

[0007] In this setting, the chemical components of the forged steel can improve the strength and toughness of the forged steel, making the forged steel have good mechanical properties.

[0008] Further, the yield strength of the ultra-high strength and large-thickness forged steel is 1016 MPa - 1086 MPa, the tensile strength is 1107 MPa - 1223 MPa, the elongation after fracture is 16.0% - 18.0%, and the impact energy absorbed at -20°C is 113 J - 193 J.

[0009] Further, the metallographic structure of the ultra-high strength and large-thickness forged steel is tempered sorbite structure and ferrite structure.

[0010] A preparation method of an ultra-high strength and large-thickness forged steel as described above, including the following steps:

[0011] Step S1: Prepare raw materials according to the chemical composition of the ultra-high strength and large-thickness forged steel, and the raw materials are successively smelted in an alkaline electric arc furnace, refined in an LF refining furnace and degassed in a VD vacuum degassing furnace to obtain a consumable electrode;

[0012] Step S2: Remelt the consumable electrode by an argon-protected electroslag furnace to obtain an electroslag ingot;

[0013] Step S3: Forge the electroslag ingot into shape;

[0014] Step S4: Heat-treat the blank by normalizing first and then annealing to obtain the blank;

[0015] Step S5: Quench and temper the blank to obtain the ultra-high strength and large-thickness forged steel.

[0016] The raw materials are formulated according to the chemical composition of ultra-high strength and large thickness forged steel, and the raw materials are successively smelted in an alkaline electric arc furnace, refined in an LF refining furnace, degassed in a VD vacuum degassing unit, remelted by electroslag remelting, forged, heat-treated, quenched, and tempered. The obtained ultra-high strength and large thickness forged steel has excellent mechanical properties and can meet the manufacturing requirements of the pressure-resistant structure of deep-sea equipment.

[0017] Further, in the step S1, the tapping temperature of the alkaline electric arc furnace is 1650 - 1680 °C, the tapping temperature of the VD vacuum degassing refining is controlled at 1625 - 1660 °C, and the gas contents in the steel of the VD vacuum degassing are [O] ≤ 0.003%, [N] ≤ 0.005%, and [H] ≤ 0.00015% respectively.

[0018] Further, in the step S2, the voltage is controlled at 40 - 70 V, the current is controlled at 7 - 15 KA, the melting rate is controlled at 0.7 - 1.7 t / h; the feeding time ≥ 2.5 h.

[0019] Further, in the step S3, the forging temperature is 800 - 1250 °C, the total forging ratio ≥ 5, and the number of forging heats ≥ 2.

[0020] Further, in the step S4, the holding temperature during normalizing is 880 - 920 °C, the holding time is 8 h, and the cooling method is air cooling; the holding temperature during annealing is 600 - 640 °C, the holding time is 12 h, and the cooling method is furnace cooling.

[0021] Further, in the step S5, the quenching treatment includes primary quenching and secondary quenching. The holding temperature during primary quenching is 860 - 900 °C; the holding temperature during secondary quenching is 680 - 760 °C. The holding time t during both primary quenching and secondary quenching is determined by the thickness h of the forged steel, t = 1.5 - 4 h, where t is in minutes and h is in millimeters.

[0022] Further, in the step S5, the tempering treatment includes a holding temperature of 560 - 620 °C during tempering, and the holding time t during tempering is determined by the thickness h of the forged steel, t = 4 - 8 h, where t is in minutes and h is in millimeters.

[0023] Compared with the prior art, the ultra-high strength and large thickness forged steel and its preparation method of the present invention have the following advantages:

[0024] (1) For the ultra-high strength and large thickness forged steel of the present invention, the chemical components of the forged steel can improve the strength and toughness of the forged steel, making the forged steel have good mechanical properties.

[0025] (2) The preparation method of the ultra-high strength and large thickness forged steel of the present invention involves formulating raw materials according to the chemical composition of the ultra-high strength and large thickness forged steel. The raw materials are successively smelted in an alkaline electric arc furnace, refined in an LF refining furnace, degassed in a VD vacuum degassing unit, remelted by electroslag remelting, forged, heat-treated, quenched, and tempered to obtain the ultra-high strength and large thickness forged steel, which has a metallographic structure of tempered sorbite and ferrite. As Figure 1-2 shown, the grain sizes are all grade 8.0 and are relatively uniform;

[0026] (3) The thickness of the ultra-high strength and large thickness forged steel of the present invention can reach 260 mm, with a yield strength of 1016 MPa to 1086 MPa, a tensile strength of 1107 MPa to 1223 MPa, an elongation after fracture of 16.0% to 18.0%, and an impact energy absorption at -20 °C of 113 J to 193 J. It has good mechanical properties and can meet the manufacturing requirements of the pressure-resistant structure of deep-sea equipment. Brief Description of the Drawings

[0027] Figure 1 is the metallographic structure diagram of the ultra-high strength and large thickness forged steel prepared in Example 3 of the present invention;

[0028] Figure 2 is the grain size diagram of the ultra-high strength and large thickness forged steel prepared in Example 3 of the present invention;

[0029] Figure 3 is the physical diagram of the ultra-high strength and large thickness forged steel during the preparation process in Example 3 of the present invention;

[0030] Figure 4 is the physical diagram of the ultra-high strength and large thickness forged steel obtained by the preparation in Example 3 of the present invention;

[0031] Figure 5 is the physical diagram of the forged steel during the preparation process in Comparative Example 5 of the present invention;

[0032] Figure 6 is the physical diagram of the forged steel during the preparation process in Comparative Example 5 of the present invention. Detailed Embodiments

[0033] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention will be given with reference to the accompanying drawings.

[0034] An ultra-high strength and large thickness forged steel, the components of which include, by weight percentage: C 0.08% to 0.18%, Mn 0.80% to 1.50%, Ni 7.0% to 10.0%, Cr 0.60% to 1.60%, Mo 0.50% to 1.20%, Si ≤ 0.40%, V ≤ 0.15%, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and unavoidable impurities.

[0035] In the forged steel composition of the present invention, the reasons for limiting the main alloying elements are described separately as follows:

[0036] Carbon (C) is the most important solid solution strengthening element, which can significantly improve the strength of forged steel, but is unfavorable to the impact toughness of the steel and also significantly reduces the weldability of the steel. Considering comprehensively, the carbon content range of the forged steel in the present invention is limited to 0.08% - 0.18%;

[0037] Silicon (Si) is one of the necessary elements for steelmaking deoxidation and also has a certain strengthening effect. However, too high a silicon content will reduce the cleanliness of the steel, and the toughness and weldability will become worse. Considering comprehensively, the silicon content range of the forged steel in the present invention is limited to not higher than 0.40%;

[0038] Manganese (Mn) is an effective element to improve strength and hardenability, but manganese has a high segregation tendency, so its content should not be too high. To ensure the hardenability of the forged steel and avoid serious segregation, the manganese content range of the forged steel in the present invention is limited to 0.8% - 1.5%;

[0039] Nickel (Ni) can improve the strength of the steel and at the same time can improve the low-temperature toughness of the steel. However, too high a nickel content will increase the production cost and also cause retained austenite in the steel, reducing the strength of the steel. The nickel content range of the forged steel in the present invention is limited to 7.0% - 10.0%;

[0040] Chromium (Cr) can play a significant solid solution strengthening role, can significantly improve the hardenability of the steel, and can improve the corrosion resistance of the steel. However, a higher chromium content will reduce the plasticity and toughness and weldability of the steel. The chromium content range of the forged steel in the present invention is limited to 0.50% - 1.20%;

[0041] Molybdenum (Mo) is a substitutional solid solution alloying element. When dissolved in austenite, it can improve the hardenability of the steel. However, too high a molybdenum content will deteriorate the plasticity and toughness of the steel. The molybdenum content range of the forged steel in the present invention is limited to 0.50% - 1.20%;

[0042] Vanadium (V) can prevent the growth of austenite grains during heat treatment and improve the toughness of the steel; vanadium dissolved in austenite can improve the hardenability of the steel, and the precipitated carbonitride particles have a precipitation strengthening effect. However, excessive vanadium will deteriorate the plasticity and toughness. The vanadium content range of the forged steel in the present invention is limited to not higher than 0.15%;

[0043] Phosphorus (P) and sulfur (S) are common impurity elements in steel. Phosphorus is prone to segregate at grain boundaries, reducing the toughness of the steel; sulfur is easy to form inclusions with other elements, reducing the strength-toughness matching. The phosphorus content of the forged steel in the present invention is limited to not higher than 0.015% and the sulfur content is limited to not higher than 0.010%.

[0044] A preparation method of ultra-high strength and large thickness forged steel, which is used to prepare the ultra-high strength and large thickness forged steel described above, includes the following steps:

[0045] Step S1: Prepare raw materials according to the chemical composition of ultra-high strength and large thickness forged steel. The raw materials are successively smelted in an alkaline electric arc furnace, refined in an LF refining furnace, and subjected to VD vacuum degassing refining to obtain consumable electrodes;

[0046] Step S2: Remelt the consumable electrodes using an argon-protected electroslag furnace to obtain electroslag ingots;

[0047] Step S3: Forge the electroslag ingots into shape;

[0048] Step S4: Perform a heat treatment process of normalizing first and then annealing on the billet to obtain a billet with uniform structure;

[0049] Step S5: Perform quenching treatment and tempering treatment on the billet in sequence to obtain forged steel with ferrite structure and martensite structure.

[0050] In the step S1, the tapping temperature of the alkaline electric arc furnace is 1650 - 1680 °C, the tapping temperature of VD vacuum degassing refining is controlled at 1625 - 1660 °C, and the gas content in the refined steel is controlled at [O] ≤ 0.003%, [N] ≤ 0.005%, [H] ≤ 0.00015%.

[0051] In the step S2, the voltage is controlled at 40 - 70 V, the current is controlled at 7 - 15 KA, the melting rate is controlled at 0.7 - 1.7 t / h; the feeding time ≥ 2.5 h.

[0052] In the step S3, the forging temperature is 800 - 1250 °C, the total forging ratio ≥ 5, and the number of forging heats ≥ 2.

[0053] In the step S4, the holding temperature during normalizing is 880 - 920 °C, the holding time is 8 h, and the cooling method is air cooling; the holding temperature during annealing is 600 - 640 °C, the holding time is 12 h, and the cooling method is furnace cooling.

[0054] In the step S5, the quenching treatment includes primary quenching and secondary quenching. The holding temperature during primary quenching is 860 - 900 °C, and the holding time is; the holding temperature during secondary quenching is 680 - 760 °C, and the holding time t is determined by the thickness h of the forged steel, t = 1.5 - 4 h, where the unit of t is min and the unit of h is mm.

[0055] In the tempering treatment in the step S5, the holding temperature during tempering is 560 - 620 °C, and the holding time t is determined by the thickness h of the forged steel, t = 4 - 8 h, where the unit of t is min and the unit of h is mm..

[0056] In the quenching treatment and tempering treatment in step S5, the cooling method is water cooling for both.

[0057] In the preparation method of an ultra-high strength and large thickness forged steel of the present invention, the reasons for defining the main process parameters are described as follows respectively:

[0058] In order to ensure the low-temperature toughness of the forged steel, the present invention processes the raw materials of the forged steel by adopting the processes of basic electric arc furnace smelting, LF refining furnace, and VD vacuum degassing refining. First, the molten steel is roughly smelted by a basic electric arc furnace to obtain a chemical composition close to the target composition, and the tapping temperature is 1650°C to 1680°C; desulfurization and fine adjustment of the composition are carried out through LF refining furnace refining, and vacuum degassing is carried out through VD vacuum degassing refining to make the gas contents such as [O], [N], and [H] in the forged steel meet [O] ≤ 0.003%, [N] ≤ 0.005%, [H] ≤ 0.00015%, and the tapping temperature is controlled at 1625°C to 1660°C, so as to ensure the low-temperature toughness of the forged steel in turn.

[0059] In order to ensure the quality of the electroslag ingot, vacuum consumable electrode casting is adopted, which can ensure that the molten steel is not exposed and improve the quality of the electroslag ingot; adopting a gas shielded electroslag furnace for consumable electrode remelting can further improve the purity of the electroslag ingot, reduce segregation more, and make the composition more uniform. The consumable electrode remelting adopts decreasing power control, the voltage is controlled at 40 - 70V, the current is controlled at 7 - 15KA, the melting rate is controlled at 0.7 - 1.7t / h, and the feeding time ≥ 2.5h.

[0060] In order to ensure the quality of the forged steel during forging and forming, the electroslag ingot needs processes such as drawing out, upsetting, forming, and correction to be forged and formed. If the forging temperature is too high, it is easy to cause the grains of the forged billet to be coarse, and even overburning and decarburization may occur, resulting in the inability to guarantee the product performance; if the forging temperature is too low, it will increase the deformation resistance, thus increasing the forging difficulty, and even cracking may occur; in order to ensure sufficient deformation of the electroslag ingot, the forging ratio of the electroslag ingot should be as large as possible. Therefore, in order to ensure the forging quality, the forging temperature is set at 800 - 1250°C, the total forging ratio should not be less than 5, and the number of forging heats should be no less than 2 times.

[0061] In order to further improve the internal uniformity of forged steel after forging, the grains are coarse and uneven after forging, and the internal stress is also relatively large. It is necessary to refine the grains through normalizing heat treatment. During the normalizing process, the forged steel needs to be fully austenitized. If the temperature is too high, it is easy to cause the grains to grow again. If the temperature is too low, it is easy to enter the two-phase region, affecting the fine-grain effect. Based on experimental analysis, the normalizing holding temperature of the forged steel is 880-920°C, the holding time is 8h, and the cooling method is air cooling; during the air cooling process of the forged steel, the inconsistent cooling rate from the surface to the core is easy to cause stress concentration, and air cooling will result in relatively high strength of the forged steel, which is not conducive to rough machining. Based on experimental analysis, annealing is carried out at 600-640°C after normalizing, the holding time is 12h, and the cooling method is furnace cooling; after post-forging heat treatment, the forged steel is rough machined to the target size.

[0062] In order to ensure greater uniformity of the forged steel after forging, during the first quenching process, the forged steel needs to be held at full austenitization. The holding temperature, holding time, and cooling method are particularly important; because, too high a temperature will cause coarse grains, affecting the final properties, and too low a temperature will cause the electroslag ingot forged steel not to be fully austenitized, forming a mixed structure of ferrite and austenite; the holding time should ensure that the forged steel can complete austenitization; the cooling method has an important impact on the final structure. Water cooling can make the forged steel form a single martensite structure, and through subsequent tempering, achieve a combination of strength and toughness. Based on calculation analysis of the steel composition and heat treatment process experiments, the holding temperature during the first quenching process is set at 850-890°C, the holding time t is determined by the thickness h of the forged steel, t = 1.5-4h, the unit of t is min, the unit of h is mm, and water cooling is used.

[0063] In order to further ensure the strength and toughness of the forged steel, during the second quenching process, the forged steel needs to enter the "ferrite + austenite" two-phase region for holding. Too high a temperature will result in a relatively high content of austenite structure and a relatively low content of ferrite, with ultimately high strength and low low-temperature toughness; the holding time should ensure that the forged steel can complete austenitization; too low a temperature will result in a relatively high content of ferrite structure and a relatively low content of austenite, with ultimately low strength; the holding time should ensure the uniformization of the forged steel structure; the cooling method has an important impact on the final structure. Water cooling can make the forged steel form a "ferrite + martensite" structure, and through subsequent tempering, achieve a combination of strength and toughness. Based on calculation analysis of the forged steel composition and heat treatment process experiments, the holding temperature during the second quenching process is set at 700-760°C, the holding time t is determined by the thickness h of the forged steel, t = 1.5-4h, the unit of t is min, the unit of h is mm, and water cooling is used.

[0064] In order to further ensure the strength and toughness of the forged steel, during the tempering process, if the tempering temperature is too high, the strength of the forged steel will be low; if the tempering temperature is too low, the strength of the forged steel will be high while the low-temperature toughness will be low; if the tempering time is short, the carbides in the martensite cannot precipitate sufficiently; if the tempering time is long, the precipitated C carbides will grow, which has an adverse effect on the performance; if the cooling rate after tempering is too slow, it will cause a large difference in the cooling rates of different parts of the surface and the core of the forged steel, and temper brittleness is likely to occur in the positions with slower cooling rates, affecting the low-temperature toughness of the steel. Through calculation and analysis in combination with the composition of the steel and heat treatment process tests, the holding temperature during the tempering process is 560 - 620 °C, the holding time t is determined by the thickness h of the forged steel, t = 4 - 8h, the unit of t is min, the unit of h is mm, and water cooling is used.

[0065] The present invention will be described in detail below in conjunction with the embodiments.

[0066] Embodiment 1

[0067] A specific embodiment of the present invention is to prepare a high-strength large-thickness forged steel with a thickness of 260 mm, which specifically includes the following steps:

[0068] Raw materials are prepared according to the chemical composition of the ultra-high-strength large-thickness forged steel. The chemical composition (weight percentage) of the ultra-high-strength large-thickness forged steel is: C 0.08, Si 0.36, Mn 1.23, Cr 1.16, Ni 7.43, Mo 0.52, V 0.14, P < 0.005, S < 0.005, O < 0.001, N 0.0038, H < 0.0001, and the balance is Fe and inevitable impurities.

[0069] The raw materials are first smelted in an alkaline electric arc furnace, refined in an LF refining furnace, and vacuum degassed in a VD vacuum degassing furnace to obtain a consumable electrode; among them, the tapping temperature in the alkaline electric arc furnace is 1650 °C, and the tapping temperature in the VD vacuum refining furnace is 1625 °C;

[0070] The consumable electrode is remelted by an argon-protected electroslag furnace to obtain an electroslag ingot. Among them, the voltage is controlled at 40 V, the current is controlled at 7 KA, and the melting rate is controlled at 0.7 t / h; the feeding time is 2.5 h;

[0071] The electroslag ingot is forged into a blank. Among them, the number of forging heats is 4 times, the forging temperature is 978 - 1189 °C, and the total number of forging passes is 6 times;

[0072] The blank is heat-treated by normalizing first and then annealing. Among them, during normalizing, 920 °C is used and the holding time is 8 h; during annealing, 600 °C is used and the holding time is 12 h to obtain a blank with uniform structure;

[0073] The blank is subjected to quenching treatment and tempering treatment. The quenching treatment includes primary quenching and secondary quenching. The temperature of primary quenching is 900 °C, and the holding time is 12 h. The temperature of secondary quenching is 680 °C, and the holding time is 12 h. The tempering temperature is 620 °C, and the holding time is 24 h. The metallographic structure of the ultra-high-strength and large-thickness forged steel obtained is ferrite structure and martensite structure, and its mechanical properties are shown in Table 1.

[0074] Example 2

[0075] A specific embodiment of the present invention is to prepare a 260-mm high-strength and large-thickness forged steel, which specifically includes the following steps:

[0076] Raw materials are prepared according to the chemical composition of the ultra-high-strength and large-thickness forged steel. The chemical composition (weight percentage) of the ultra-high-strength and large-thickness forged steel is: C 0.15, Si 0.26, Mn 1.05, Cr 0.83, Ni 8.62, Mo 0.92, V 0.09, P < 0.005, S < 0.005, O < 0.001, N 0.0025, H < 0.0001, and the balance is Fe and unavoidable impurities.

[0077] The raw materials are first smelted in an alkaline electric arc furnace, refined in an LF refining furnace, and degassed in a VD vacuum degassing furnace to obtain a consumable electrode. The tapping temperature in the alkaline electric arc furnace is 1660 °C, and the tapping temperature in the VD vacuum degassing furnace is 1645 °C.

[0078] The consumable electrode is remelted by an argon-protected electroslag furnace to obtain an electroslag ingot. Among them, the voltage is controlled at 50 V, the current is controlled at 12 KA, and the melting rate is controlled at 1.2 t / h; the feeding time is 3.5 h.

[0079] The electroslag ingot is forged into a blank. Among them, the number of forging heats is 4 times, the forging temperature is 978 - 1189 °C, and the total forging times is 6 times.

[0080] The blank is first normalized and then annealed. During normalization, 900 °C is used and the holding time is 8 h. During annealing, 620 °C is used and the holding time is 12 h to obtain a blank with uniform structure.

[0081] The blank is subjected to quenching treatment and tempering treatment. The quenching treatment includes primary quenching and secondary quenching. The temperature of primary quenching is 860 °C, and the holding time is 5 h. The temperature of secondary quenching is 740 °C, and the holding time is 5 h. The tempering temperature is 600 °C, and the holding time is 15 h to obtain a blank with ferrite structure and martensite structure, and its mechanical properties are shown in Table 1.

[0082] Example 3

[0083] A specific embodiment of the present invention for preparing a high-strength and large-thickness forged steel with a thickness of 260 mm specifically includes the following steps:

[0084] Prepare raw materials according to the chemical composition of the ultra-high-strength and large-thickness forged steel. The chemical composition (by weight percentage) of the ultra-high-strength and large-thickness forged steel is: C 0.18, Si 0.13, Mn 0.83, Cr 1.55, Ni 9.75, Mo 1.14, V 0.09, P < 0.005, S < 0.005, O < 0.001, N 0.0029, H < 0.0001, and the balance is Fe and unavoidable impurities.

[0085] The raw materials are first smelted in an alkaline electric arc furnace, refined in an LF refining furnace, and degassed in a VD vacuum degassing refining furnace to obtain consumable electrodes; among them, the tapping temperature in the alkaline electric arc furnace is 1680 °C, and the tapping temperature in the VD vacuum degassing refining furnace is 1660 °C;

[0086] The consumable electrodes are remelted by an argon-protected electroslag furnace to obtain electroslag ingots. Among them, the voltage is controlled at 70 V, the current is controlled at 15 KA, and the melting rate is controlled at 1.7 t / h; the feeding time is 4.5 h;

[0087] The electroslag ingots are forged into blanks. Among them, the number of forging heats is 4 times, the forging temperature is 978 - 1189 °C, and the total forging ratio is 6 times;

[0088] The blanks are heat-treated by normalizing first and then annealing. Among them, the normalizing temperature is 880 °C, the holding time is 8 h, the annealing temperature is 640 °C, and the holding time is 12 h to obtain blanks with uniform structure;

[0089] The blanks are quenched and tempered. Among them, the quenching treatment includes primary quenching and secondary quenching. The temperature of primary quenching is 880 °C, the holding time is 12 h, the temperature of secondary quenching is 760 °C, the holding time is 12 h, the tempering temperature is 560 °C, and the holding time is 24 h to obtain blanks with ferrite and martensite structures, and their mechanical properties are shown in Table 1.

[0090] Example 4

[0091] The differences between this example and Example 3 are as follows:

[0092] The thickness of the prepared ultra-strong and large-thickness forged steel is 60 mm, where the forging temperature is 813 - 1128 °C, the number of forging times is 2 times, and the total forging ratio is 12, and its mechanical properties are shown in Table 1.

[0093] Example 5

[0094] The differences between this example and Example 3 are as follows:

[0095] The thickness of the prepared super high-strength thick forged steel is 120 mm, the forging temperature is 952 - 1203 °C, the number of forging times is 3, the total forging ratio is 5, and its mechanical properties are shown in Table 1.

[0096] Example 6

[0097] The difference between this example and Example 3 is as follows:

[0098] The thickness of the prepared super high-strength thick forged steel is 200 mm, the forging temperature is 908 - 1243 °C, the number of forging times is 3, the total forging ratio is 8, and its mechanical properties are shown in Table 1.

[0099] Comparative Example 1

[0100] The difference between this comparative example and Example 3 is as follows;

[0101] The heat treatment method of directly annealing the blank is adopted, and normalizing treatment is no longer carried out. The annealing is carried out at 620 °C for 12 h to obtain forged steel with uniform structure, and its mechanical properties are shown in Table 1;

[0102] Comparative Example 2

[0103] The difference between this comparative example and Example 3 is as follows;

[0104] The blank is quenched and tempered. The quenching treatment only includes one quenching, the quenching temperature is 880 °C, the holding time is 12 h, the tempering temperature is 570 °C, and the holding time is 24 h. The obtained forged steel has mechanical properties shown in Table 1.

[0105] Comparative Example 3

[0106] The difference between this comparative example and Example 3 is as follows:

[0107] Raw materials are prepared according to the chemical composition of super high-strength thick forged steel. The chemical composition (weight percentage) of super high-strength thick forged steel is: C 0.16, Si 0.23, Mn 0.18, Cr 0.65, Ni 9.5, Mo 1.08, V 0.08, P < 0.005, S < 0.005, O 0.0064, N 0.0056, H < 0.0001, and the balance is Fe and inevitable impurities.

[0108] The forging temperature is 886 - 1143 °C, the number of forging times is 5, the total forging ratio is 6, and the mechanical properties of the obtained forged steel are shown in Table 1.

[0109] Comparative Example 4

[0110] The difference between this comparative example and Example 3 is as follows:

[0111] Prepare raw materials according to the chemical composition of ultra-high strength and large thickness forged steel. The chemical composition (weight percentage) of ultra-high strength and large thickness forged steel is: C 0.20, Si 0.15, Mn 0.87, Cr 1.7, Ni 1.0, Mo 1.20, V 0.10, P 0.005, S 0.005, O 0.001, N 0.0035, H 0.0001, and the balance is Fe and inevitable impurities. The mechanical properties are shown in Table 1.

[0112] Comparative Example 5

[0113] The difference between this comparative example and Example 3 is as follows:

[0114] Among them, the forging temperature is 736 - 1053 °C, the number of forging times is 4 times, and the total forging ratio is 6.

[0115] Table 1 compares the mechanical properties of Examples 1 - 6 and Comparative Examples 1 - 4

[0116]

[0117]

[0118] The ultra-high strength and large-thickness forged steel obtained in Examples 1-3 has excellent mechanical properties. When the thickness is 260 mm, the yield strength of the forged steel is 1016 MPa - 1086 MPa, the tensile strength is 1137 MPa - 1223 MPa, the elongation after fracture is 16.0% - 18.0%, and the impact energy absorption at -20 °C is 113 J - 193 J. According to Examples 4-6, the ultra-high strength and large-thickness forged steel of the present invention is also applicable to forged steel with a thickness less than 260 mm and has good mechanical properties. Among them, for a 200-mm wall-thickness forged steel part, the reduction in yield strength from the surface to the core is within 60 MPa, which is relatively uniform. From Example 3 and Comparative Example 1, it can be seen that normalizing treatment in the heat treatment process can significantly improve the mechanical properties of the forged steel. From Example 3 and Comparative Example 2, it can be seen that by secondary quenching and the temperature of the secondary quenching being lower than that of the primary quenching, the mechanical properties of the forged steel can be significantly improved. From Example 3 and Comparative Example 3, it can be seen that due to the higher gas content in Comparative Example 3, the impact energy absorption of Comparative Example 3 is low, that is, within a certain range of gas content of the present invention, the mechanical properties of the forged steel can be significantly improved. From Example 3 and Comparative Example 4, it can be seen that within a certain range of the chemical composition of the forged steel raw materials, the mechanical properties of the forged steel can be significantly improved. In short, according to the chemical composition of the forged steel of the present invention, the raw materials are smelted in an electric arc furnace, refined in an LF refining furnace, degassed in a VD vacuum degassing unit, remelted by electroslag remelting, forged, heat-treated, quenched, and tempered in sequence. The obtained ultra-high strength and large-thickness forged steel has a yield strength of 1016 MPa - 1086 MPa, a tensile strength of 1107 MPa - 1223 MPa, an elongation after fracture of 16.0% - 18.0%, and an impact energy absorption at -20 °C of 113 J - 193 J; the ultra-high strength and large-thickness forged steel has good mechanical properties and can meet the manufacturing requirements of the pressure-resistant structure of deep-sea equipment.

[0119] As Figure 3-4 shown, the forged steel prepared in Example 3 shows no cracks, inclusions, etc., and has good forging quality. As Figure 5-6 shown, in Comparative Example 5, due to the relatively low forging temperature, microcracks are generated on the surface of the forged steel.

[0120] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be subject to the scope defined by the claims.

Claims

1. An ultra-high strength and large thickness forged steel, characterized in that, The yield strength of the ultra-high strength and large thickness forged steel is 1016 MPa to 1086 MPa, the tensile strength is 1107 MPa to 1223 MPa, the elongation after fracture is 16.0% to 18.0%, the impact energy absorbed at -20 °C is 113 J to 193 J, and the maximum thickness can reach 260 mm; the metallographic structure of the ultra-high strength and large thickness forged steel is tempered martensite structure and ferrite structure; the chemical composition of the forged steel by weight percentage includes: C 0.08% to 0.18%, Mn 0.80% to 1.50%, Ni 7.0% to 10.0%, Cr 0.60% to 1.60%, Mo 0.50% to 1.20%, Si ≤ 0.40%, V ≤ 0.15%, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and inevitable impurities; The preparation method of the ultra-high strength and large thickness forged steel described above includes the following steps: Step S1: Prepare raw materials according to the chemical composition of the ultra-high strength and large thickness forged steel. The raw materials are successively smelted in an alkaline electric arc furnace, refined in an LF refining furnace, and vacuum degassed in a VD vacuum degassing furnace to obtain a consumable electrode; Step S2: Remelt the consumable electrode by an argon-protected electroslag furnace to obtain an electroslag ingot; Step S3: Forge and shape the electroslag ingot; Step S4: Heat treat the blank by normalizing first and then annealing to obtain the blank; Step S5: Quench and temper the blank to obtain the ultra-high strength and large thickness forged steel.

2. The ultra-high strength and large thickness forged steel according to claim 1, characterized in that, In the step S1, the tapping temperature of the alkaline electric arc furnace is 1650 to 1680 °C, the tapping temperature of the VD vacuum degassing refining is controlled at 1625 to 1660 °C, and the weight percentage content of gases in the steel of the VD vacuum degassing is [O] ≤ 0.003%, [N] ≤ 0.005%, [H] ≤ 0.00015% respectively.

3. An ultra-high strength and large thickness forged steel according to claim 1, characterized in that, In the step S2, the voltage is controlled at 40 to 70 V, the current is controlled at 7 to 15 KA, and the melting rate is controlled at 0.7 to 1.7 t / h; the feeding time ≥ 2.5 h.

4. An ultra-high strength and large-thickness forged steel according to claim 1, characterized in that, In the step S3, the forging temperature is 800 to 1250 °C, the total forging ratio ≥ 5, and the number of forging heats ≥ 2.

5. An ultra-high strength and large thickness forged steel according to claim 1, characterized in that, In the step S4, the holding temperature during normalizing is 880 to 920 °C, the holding time is 8 h, and the cooling method is air cooling; the holding temperature during annealing is 600 to 640 °C, the holding time is 12 h, and the cooling method is furnace cooling.

6. The ultra-high strength and large thickness forged steel according to claim 1, wherein, In the step S5, the quenching treatment includes primary quenching and secondary quenching. The holding temperature during primary quenching is 860 to 900 °C; the holding temperature during secondary quenching is 680 to 760 °C. The holding time t during primary quenching and secondary quenching is determined by the thickness h of the forged steel, t = 1.5 to 4 h, the unit of t is min, and the unit of h is mm.

7. An ultra-high-strength and large-thickness forged steel according to claim 1, characterized in that, In the step S5, the tempering treatment includes the holding temperature during tempering of 560 to 620 °C. The holding time t during tempering is determined by the thickness h of the forged steel, t = 4 to 8 h, the unit of t is min, and the unit of h is mm.

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