A high-strength and high-toughness ship hull structural steel for extremely cold environments, its preparation method and application

Abstract

This invention provides a high-strength and high-toughness ship hull structural steel for extremely cold environments, its preparation method, and its application, belonging to the field of high-strength and high-toughness steel metallurgical technology. This invention achieves the goal of "step-by-step, temperature-zone-separated" hierarchical construction of the multi-element and trace element system (rare earth elements LaCe, Al, Nb, V, Ti) in the ship hull structural steel by precisely designing the multi-element and trace element system (rare earth elements LaCe, Al, Nb, V, Ti) during smelting, adding cored composite wires, and controlling the multi-element and trace element system to construct multi-element nanophases in the high-strength and high-toughness steel through reasonable process control. This allows for the regulation of the strength and toughness of the ship hull structural steel, especially reducing the yield strength ratio and improving the -84℃ low-temperature impact toughness of the ship hull structural steel and its welds, meeting the requirements for use in extremely cold environments.

Description

Technical Field

[0001] This invention relates to the field of high-strength and high-toughness steel metallurgy technology, and in particular to a high-strength and high-toughness ship hull structural steel for extremely cold environments, its preparation method, and its application. Background Technology

[0002] In recent years, with the rapid development of major equipment and its increasingly harsh service environment, significant demands have been placed on the service life of new-generation high-end weaponry and equipment, including ships, armored vehicles, and aerospace equipment. Ships, in particular, operate in extremely harsh environments—long-term exposure to complex marine conditions, especially the extreme temperature variations in polar regions, and enduring strong impacts for much of the time. These complex factors severely impact their service safety. Therefore, the steel used in shipbuilding equipment must possess excellent comprehensive properties, requiring not only high strength but also good ductility and toughness to ensure safety and excellent service capability. Currently, the technological approach for producing steel for shipbuilding equipment still employs the traditional high-carbon, high-alloy composition + quenching and tempering heat treatment process. While this technology can produce hull structural steel that meets requirements in service environments above -50°C, it is not readily adaptable to modern needs. First, the high carbon and high alloy content severely affects plasticity and toughness, making the forming process extremely complex. It requires pre-welding heat treatment and post-welding heat treatment, increasing the number of processes, time, and costs. Second, its microstructure is mainly composed of tempered martensite and other single structures. The steel plate has a high yield strength ratio (0.96-0.98) and low low-temperature toughness. Since the yield strength of the material is close to the failure strength, it is prone to sudden brittle fracture in extremely cold marine environments (-50 to -84℃), which will lead to poor safety and reliability and a short service life.

[0003] Currently, my country's high-carbon, high-alloy steel technology mainly imitates and adopts the HY series steel from abroad. HY series steel relies primarily on high carbon and high alloy content to improve its strength, but this leads to serious problems in its forming. During the construction of the USS Seawolf, the United States experienced severe cracking. To address the resulting problems of complex forming processes, long manufacturing cycles, and high costs, the US began developing HSLA series steel in the 1970s and 80s. HSLA series steel primarily improves its formability by significantly reducing carbon and alloy content. The resulting decrease in strength is compensated for by nano-Cu particles precipitated during aging. It is gradually replacing HY series steel. However, during use, it has been found that HSLA series steel also has some technical defects. For example, the Cu nano-precipitates generated during aging begin to coarsen and grow at temperatures exceeding a certain level (>550℃), evolving into irregular shapes (elongated strips or rods, etc.). Simultaneously, the coherent relationship with the matrix gradually transforms into incoherence, i.e., coherence is lost, thus severely weakening the strengthening effect of Cu nanoparticles and significantly impacting the ductility and toughness of HSLA series steel.

[0004] Therefore, researching high-strength, high-toughness, and high-performance ship structural steel remains of great significance. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength and high-toughness hull structural steel for extremely cold environments, its preparation method and application, wherein the hull structural steel has the advantages of high strength and toughness, high and low temperature toughness and low yield strength ratio.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing high-strength and high-toughness ship hull structural steel for extremely cold environments, comprising the following steps:

[0008] The raw materials corresponding to the main chemical composition of high-strength and tough ship hull structural steel are mixed and vacuum melted. The resulting molten steel is fed into a cored composite wire and then refined and cast in sequence to obtain steel ingots.

[0009] The steel ingot is forged, rolled and heat-treated in sequence to obtain high-strength and high-toughness ship hull structural steel for extremely cold environments;

[0010] The main chemical composition of the high-strength and high-toughness ship hull structural steel includes: C 0.03-0.07%, Si 0.03-0.045%, Mn 0.80-1.10%, Ni 3.5-5.5%, Cr 0.35-0.55%, Mo 0.40-0.65%, Al 0.015-0.03%, V 0.03-0.08%, Nb 0.03-0.06%, Ti 0.005-0.02%, P≤0.01%, S≤0.003%, balance Fe;

[0011] The chemical composition of the cored composite wire, by mass percentage, is: LaCe 2-5%, Mg 4-9%, Ti 0.1-0.3%, Ca 3-6%, Ba 3-6%, with the balance being Fe; the mass ratio of La to Ce is 1:1.

[0012] Preferably, the cored composite wire is made of iron sheet covered with a metal core material; the metal core material includes Mg wire, LaCe composite rare earth wire, Ti wire and calcium barium powder; the diameter of the cored composite wire is 8 to 13.5 mm.

[0013] Preferably, the diameter of the Mg wire is 1-3 mm; the diameter of the LaCe composite rare earth wire is 1.5-3.5 mm; the diameter of the Ti wire is 1-2.5 mm; the particle size of the calcium barium powder is 0.1-2.5 mm; and the wall thickness of the iron sheet is 0.1-0.2 mm.

[0014] Preferably, the oxygen content of the LaCe composite rare earth fiber is <50ppm.

[0015] Preferably, the feeding speed of the core-spun composite wire is 0.05 to 1.5 m / s; the feeding weight of the core-spun composite wire is 0.015 to 1.0% of the total weight of the raw materials.

[0016] Preferably, the refining power is 80-160KW.

[0017] Preferably, the heat treatment includes sequential quenching, secondary quenching, and tempering.

[0018] Preferably, the temperature of the first quenching is 850-920℃, and the holding time is 1-2h; the temperature of the second quenching is 650-720℃, and the holding time is 1-2h; the temperature of the tempering is 530-580℃, and the holding time is 1-2h.

[0019] The present invention provides a high-strength and high-toughness ship hull structural steel for use in extremely cold environments prepared by the preparation method described above. The matrix structure of the high-strength and high-toughness ship hull structural steel includes two or more of ferrite, tempered sorbite and tempered martensite.

[0020] This invention provides the application of the high-strength and high-toughness hull structural steel for extremely cold environments described above in ship equipment serving in extremely cold sea environments, where the temperature of the extremely cold sea environment is -50 to -84℃.

[0021] This invention precisely designs the multi-element and trace element system (rare earth elements LaCe, Al, Nb, V, Ti) in ship hull structural steel. During smelting, a cored composite wire is added, and the multi-element and trace element composition is controlled to construct a multi-element nanophase in the high-strength and tough steel. Through rational process control, an oxidation reaction is performed in the molten steel to control the interfacial concentration gradient of nanoparticles, forming nanoscale oxides. These nanoscale oxides can achieve heterogeneous nucleation, refine the solidification structure, and serve as nucleation sites for nanoscale carbonitrides and metallic compounds formed during subsequent phase transformations and aging processes, preventing their precipitation at grain boundaries and weakening the grain boundaries. This achieves the goal of constructing a multi-element nanophase in a "step-by-step, temperature-separated" hierarchical manner, thereby regulating the strength and toughness of the ship hull structural steel, especially reducing the yield strength ratio and improving the -84℃ low-temperature impact toughness of the ship hull structural steel and its welds, resulting in high-strength and tough ship hull structural steel (yield strength R...). p0.2 ≥785MPa), meeting the requirements for use in extremely cold environments.

[0022] The preparation process of this invention is highly operable and the composition is controllable. Compared with traditional smelting and deoxidation processes, the oxides formed in the molten hull structural steel are fine and can also achieve heterogeneous nucleation. The hierarchical construction of multi-dimensional nanophases is beneficial to improving and stabilizing the strength and toughness of hull structural steel, reducing its yield strength ratio and improving its low-temperature impact toughness at -84℃, achieving a double increase in strength and toughness, and can be industrially promoted and applied. Attached Figure Description

[0023] Figure 1 Here is a schematic diagram (a) of the cored composite wire structure of the present invention and a physical image (b) of the cored composite wire used in Example 1;

[0024] Figure 2 This is a schematic diagram of the cored composite wire feeding device of the present invention;

[0025] Figure 3 Characterization diagram of the multi-component nanophases constructed in the middle layer of the high-strength and high-toughness ship hull structural steel for extreme cold environments prepared in Example 1. Detailed Implementation

[0026] This invention provides a method for preparing high-strength and high-toughness ship hull structural steel for extremely cold environments, comprising the following steps:

[0027] The raw materials corresponding to the main chemical composition of high-strength and tough ship hull structural steel are mixed and vacuum melted. The resulting molten steel is fed into a cored composite wire and then refined and cast in sequence to obtain steel ingots.

[0028] The steel ingot is forged, rolled and heat-treated in sequence to obtain high-strength and high-toughness ship hull structural steel for extremely cold environments;

[0029] The main chemical composition of the high-strength and high-toughness ship hull structural steel includes: C 0.03-0.07%, Si 0.03-0.045%, Mn 0.80-1.10%, Ni 3.5-5.5%, Cr 0.35-0.55%, Mo 0.40-0.65%, Al 0.015-0.03%, V 0.03-0.08%, Nb 0.03-0.06%, Ti 0.005-0.02%, P≤0.01%, S≤0.003%, balance Fe;

[0030] The chemical composition of the cored composite wire, by mass percentage, is: LaCe 2-5%, Mg 4-9%, Ti 0.1-0.3%, Ca 3-6%, Ba 3-6%, with the balance being Fe; the mass ratio of La to Ce is 1:1.

[0031] The present invention does not impose any special limitations on the source and specific form of the raw materials corresponding to the main chemical composition of the high-strength and tough hull structural steel. The appropriate raw materials can be selected according to conventional methods in the field.

[0032] In this invention, the raw materials are preferably added in such a way that larger pieces are placed near the inner wall of the crucible and smaller pieces are placed near the center of the crucible; the furnace lid is closed, the vacuum pump is turned on, and the vacuum degree of the melting furnace is evacuated to below 3.5 Pa; the power of the melting furnace is set to 10-180 KW (more preferably 60-90 KW, even more preferably 70-80 KW), and electricity is applied to raise the temperature. When the metal in the crucible begins to melt, the power is increased to 80-160 KW (more preferably 140-150 KW) for melting; when splashing occurs, the power is reduced to 50-100 KW (more preferably 60-80 KW), the vacuum pump is turned off, and argon gas is introduced into the furnace at the same time to increase the vacuum pressure of the furnace body to 200-300 Pa, more preferably 220-250 Pa; the ship hull structural steel raw material is added to the crucible for vacuum melting.

[0033] In this invention, the vacuum melting temperature is preferably 1600-1650°C, more preferably 1620-1640°C.

[0034] like Figure 2 As shown, in the vacuum melting process, the present invention preferably feeds cored composite wire into the molten steel in the crucible through a sealed wire feeding mechanism installed on the furnace cover of the vacuum induction furnace to deoxidize and control the oxygen content (≤10ppm). After the melt is cleared, the content of each element in the obtained steel melt is sampled and tested to ensure that the content of each element in the steel meets the composition design requirements.

[0035] like Figure 1 As shown in (a), the cored composite wire of the present invention is preferably made of a metal core material covered with iron sheet; the metal core material preferably includes Mg wire, LaCe composite rare earth wire, Ti wire and calcium barium powder; the calcium barium powder is uniformly dispersed in the gaps between the Mg wire, LaCe composite rare earth wire and Ti wire.

[0036] In this invention, the diameter of the core-coated composite wire is preferably 8 to 13.5 mm, more preferably 10 to 13 mm; the length of the core-coated composite wire is preferably determined according to the feed weight of the core-coated composite wire.

[0037] In this invention, the diameter of the Mg wire is preferably 1-3 mm, more preferably 1.5-2.5 mm, and even more preferably 2.0-2.3 mm; the diameter of the LaCe composite rare earth wire is preferably 1.5-3.5 mm, more preferably 2.0-3.0 mm; the oxygen content of the LaCe composite rare earth wire is preferably <50 ppm; and the mass ratio of La to Ce in the LaCe composite rare earth wire is preferably 1:1.

[0038] In this invention, the diameter of the Ti wire is preferably 1 to 2.5 mm, more preferably 1.5 to 2.0 mm.

[0039] In this invention, the particle size of the calcium barium powder is preferably 0.1-2.5 mm, more preferably 0.5-2.0 mm, and even more preferably 1.0-1.5 mm; the calcium barium powder is a mixture of calcium powder and barium powder.

[0040] In this invention, the wall thickness of the sheet metal is preferably 0.1 to 0.2 mm, and more preferably 0.15 mm.

[0041] The chemical composition of the cored composite wire, by mass percentage, is as follows: LaCe 2-5% (La and Ce ratio 1:1), Mg 4-9%, Ti 0.1-0.3%, Ca 3-6%, Ba 3-6%, with the balance being Fe; more preferably, Mg 5.5-7.5%, even more preferably 6.0-7.0%, calcium 4-5%, more preferably 4.5-4.8%, barium 4-5%, LaCe composite rare earth 3-4.5%, even more preferably 3.5-4%, and Ti 0.15-0.25%, more preferably 0.20%.

[0042] The present invention preferably selects iron sheet and metal core material of corresponding quality according to the chemical composition of the cored composite wire to form a cored composite wire.

[0043] The present invention does not impose any special limitations on the preparation method of the cored composite wire. It can be made by directly and completely covering the metal core material with iron sheet according to the required chemical composition.

[0044] In this invention, the feeding speed of the cored composite wire is preferably 0.05-1.5 m / s, more preferably 0.3-0.8 m / s, and even more preferably 0.55-0.65 m / s; the feeding weight of the cored composite wire is preferably 0.015-1.0% of the total weight of the raw materials, more preferably 0.018-0.03%, and even more preferably 0.02-0.25%; this invention preferably converts the feeding weight of the cored composite wire into the length and diameter of the cored composite wire, and feeds the cored composite wire of a specific diameter and length into the molten steel.

[0045] The present invention does not impose any special limitations on the melting and cleaning process; it can be carried out in accordance with methods well known in the art.

[0046] After feeding the cored composite wire, the present invention preferably turns on the vacuum pump, sets the refining power, and observes the metal raw material in the crucible through the observation hole to ensure it is clear. Refining continues until the molten steel begins to splash, at which point the power is turned off to cool down, and stirring is intensified (to promote the oxidation-reduction reaction of the molten steel and control the interfacial concentration gradient of nanoparticles in the molten steel). In this invention, the refining power is preferably 80–160 kW, more preferably 100–110 kW.

[0047] After refining, the present invention preferably involves hoisting the ingot mold and casting channel, turning on the vacuum pump, evacuating to a vacuum level below 5 Pa, supplying power at 60-100 kW (more preferably 70-80 kW), and allowing the refined melt to stand for 10-20 minutes (more preferably 15 minutes). Then, the crucible is tilted and poured using the operator's handle. This invention promotes the flotation of oxide inclusions by allowing the melt to stand, thereby improving the purity of the ship hull structural steel.

[0048] After casting, the present invention preferably cools down, opens the venting valve to break the vacuum, opens the furnace door, uses an overhead crane to move the steel ingot mold to the demolding area, disassembles the steel ingot mold, takes out the hull structure steel ingot, and performs forging, rolling and heat treatment in sequence to obtain the hull structure steel of the required specifications and dimensions.

[0049] The present invention does not impose any special limitations on the forging and rolling processes; the processes can be carried out according to methods well known in the art based on the actual required dimensions.

[0050] In this invention, the heat treatment preferably includes sequential quenching, secondary quenching, and tempering. The temperature of the primary quenching is preferably 850–920°C, more preferably 860–900°C, and even more preferably 870–880°C, with a holding time preferably 1–2 hours, and even more preferably 1.5 hours. The temperature of the secondary quenching is preferably 650–720°C, more preferably 660–700°C, and even more preferably 680–690°C, with a holding time preferably 1–2 hours, and even more preferably 1.5 hours. The temperature of the tempering is preferably 530–580°C, more preferably 540–570°C, and even more preferably 550–560°C, with a holding time preferably 1–2 hours, and even more preferably 1.5 hours. The heat treatment method of this invention can control the microstructure of high-strength and high-toughness steel.

[0051] After the heat treatment is completed, the present invention preferably air-cools the material to room temperature to obtain the hull structural steel.

[0052] In this invention, the main chemical composition of the high-strength and high-toughness ship hull structural steel includes: C 0.03-0.07%, Si 0.03-0.045%, Mn 0.80-1.10%, Ni 3.5-5.5%, Cr 0.35-0.55%, Mo 0.40-0.65%, Al 0.015-0.03%, V 0.03-0.08%, Nb 0.03-0.06%, Ti 0.005-0.02%, P≤0.01%, S≤0.003%, with the balance being Fe.

[0053] In the hull structural steel described in this invention, rare earth elements LaCe, Mg, Al, and Ti will undergo an oxidation reaction with oxygen in the steel melt. The strong Ar blowing during the smelting process generates a flow field, thereby controlling the interfacial concentration gradient of nanoparticles in the steel melt and forming nanoscale oxides. This achieves heterogeneous nucleation and refines the solidification structure. Nb, V, Ti, and Al elements will form nanoscale carbonitrides and metallic compounds during phase transformation and aging processes. These nanoscale oxides will act as nucleation cores, preventing precipitation at grain boundaries and thus weakening the grain boundaries. This improves the strength and toughness of the hull structural steel, reduces its yield strength ratio, and increases its impact toughness at -84℃.

[0054] In this invention, C is 0.03-0.07%, preferably 0.045-0.06%, more preferably 0.05-0.052%; Si is 0.03-0.045%, preferably 0.035-0.042%, more preferably 0.038-0.04%; Mn is 0.80-1.10%, preferably 0.9-1.0%, more preferably 0.95-0.98%; Ni is 3.5-5.5%, preferably 4.0-5.0%, more preferably 4.5-4.8%; Cr is 0.35-0.55%, preferably 0.40-0.50%, more preferably 0.45-0.48%; Mo is 0.40-0.65%, preferably 0.45-0.6%, more preferably 0.50-0.55%; Al 0.015–0.03%, preferably 0.018–0.028%, more preferably 0.02–0.025%; V 0.03–0.08%, preferably 0.05–0.07%, more preferably 0.055–0.06%; Nb 0.03–0.06%, preferably 0.035–0.055%, more preferably 0.045–0.05%; Ti 0.005–0.02%, preferably 0.01–0.018%, more preferably 0.013–0.016%.

[0055] The present invention provides a high-strength and high-toughness ship hull structural steel for use in extremely cold environments prepared by the preparation method described above. The matrix structure of the high-strength and high-toughness ship hull structural steel includes two or more of ferrite, tempered sorbite and tempered martensite.

[0056] This invention provides the application of the high-strength and high-toughness hull structural steel described above in naval equipment serving in extremely cold sea environments, where the temperature is -50 to -84°C. This invention does not impose any specific limitations on the application method; methods well-known in the art can be used.

[0057] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0058] In the following embodiments, the core-coated composite wire is manufactured by directly and completely covering the corresponding metal core material with iron sheet according to the required chemical composition of the core-coated composite wire.

[0059] Example 1

[0060] 1. Ingredients

[0061] Accurately weigh 200 kg per furnace; the chemical composition of the high-strength and tough hull structural steel is designed as follows: C 0.045%, Si 0.035%, Mn 1.0%, Ni 5.0%, Cr 0.4%, Mo 0.55%, Al 0.018%, V 0.06%, Nb 0.045%, Ti 0.01%, P 0.003%, S 0.0015%, balance Fe.

[0062] 2. Smelting operation

[0063] 1) Add the raw materials in such a way that the larger pieces are placed near the inner wall of the crucible and the smaller pieces are placed near the center of the crucible;

[0064] 2) Close the furnace cover, turn on the vacuum pump, and evacuate the furnace to a vacuum level of <3.5 Pa;

[0065] 3) Set the power of the melting furnace to 60KW, and heat it up. When the metal in the crucible begins to melt, increase the power to 160KW to melt it.

[0066] 4) When the melting reaches the point of splashing, reduce the power to 50KW, turn off the vacuum pump, and at the same time, fill the furnace with argon gas to increase the vacuum pressure of the furnace body to 200Pa;

[0067] 5) The raw materials for the hull structure steel are added to a crucible for vacuum melting at a temperature of 1600℃, using a cored composite wire (see actual image). Figure 1 As shown in (b), the composition is as follows: Mg 5.5%, calcium powder 4%, barium powder 4%, LaCe composite rare earth 3% (La to Ce mass ratio 1:1), Ti 0.15%, with the balance being Fe; the diameter of the Mg wire is 1.5 mm; the diameter of the LaCe composite rare earth wire is 1.5 mm, and the oxygen content of the LaCe composite rare earth wire is <50 ppm; the diameter of the Ti wire is 1 mm; the particle size of the calcium and barium powder is 0.1-0.5 mm; the wall thickness of the iron sheet is 0.1 mm. Secondary materials are added to the crucible through a sealed wire feeding mechanism installed on the furnace cover of the vacuum induction furnace. The feeding weight of the cored composite wire is 0.02% of the total weight of the raw materials. Based on the conversion between the feeding weight of the cored composite wire and the length of the composite wire, a cored composite wire with a diameter of 8 mm and a length of 7.5 m is fed into the molten steel at a speed of 0.8 m / s for deoxidation and oxygen content (≤10 ppm) control. After the steel melt is cleared, samples are taken to test the content of each element in the molten steel.

[0068] 6) Turn on the vacuum pump, set the refining power to 80KW, observe the metal raw materials in the crucible through the observation hole until they are clear, continue refining until the molten steel begins to splash, then turn off the power to cool down and strengthen the stirring of the melt.

[0069] 7) Hoist the steel ingot mold and flow channel, turn on the vacuum pump, evacuate to <5Pa, power 70KW, let stand for 20 minutes before pouring, and then operate the handle to tilt the crucible for pouring.

[0070] 8) After casting, cool down, open the venting valve to break the vacuum, open the furnace door, take out and disassemble the steel ingot mold, take out the ship structure steel ingot, and carry out billet forging and multi-pass rolling to obtain the steel ingot of the required specifications and dimensions.

[0071] 9) The steel ingot is held at 860℃ for 1 hour and then quenched. It is then heated to 680℃ for 1.5 hours and quenched again. Finally, it is tempered and heated to 560℃ for 1 hour and then air-cooled to room temperature. The resulting hull structure steel has a multi-type structure of ferrite and tempered lath martensite.

[0072] Example 2

[0073] 1. Ingredients

[0074] Accurately weigh 200 kg per furnace; the chemical composition of the high-strength and tough hull structural steel is designed as follows: C 0.05%, Si 0.04%, Mn 0.90%, Ni 4.0%, Cr 0.45%, Mo 0.50%, Al 0.02%, V 0.05%, Nb 0.035%, Ti 0.013%, P 0.004%, S 0.0018%, balance Fe.

[0075] 2. Smelting operation

[0076] 1) Add the raw materials in such a way that the larger pieces are placed near the inner wall of the crucible and the smaller pieces are placed near the center of the crucible;

[0077] 2) Close the furnace cover, turn on the vacuum pump, and evacuate the furnace to a vacuum level of <3.5 Pa;

[0078] 3) Set the power of the melting furnace to 80KW, and heat it up. When the metal in the crucible begins to melt, increase the power to 150KW to melt it.

[0079] 4) When the melting reaches the point of splashing, reduce the power to 60KW, turn off the vacuum pump, and simultaneously introduce argon gas into the furnace to increase the vacuum pressure of the furnace body to 250Pa;

[0080] 5) The raw materials for the ship hull structure steel were added to a crucible for vacuum melting at a temperature of 1620℃. Cored composite wire (Mg 6.0%, calcium powder 4.5%, barium powder 5.0%, LaCe composite rare earth 4.0% (La to Ce mass ratio 1:1), Ti 0.25%, balance Fe; Mg wire diameter 2.0mm; LaCe composite rare earth wire diameter 2.0mm, oxygen content <50ppm; Ti wire diameter 1.5mm; calcium and barium powder particle size...) The thickness is 0.5-1.0 mm; the wall thickness of the sheet metal is 0.15 mm. Secondary material is added to the crucible through the sealed wire feeding mechanism installed on the furnace cover of the vacuum induction furnace. The feeding weight of the cored composite wire is 0.025% of the total weight of the raw materials. Based on the conversion between the feeding weight of the cored composite wire and the length of the composite wire, a cored composite wire with a diameter of 10 mm and a length of 6 m is fed into the molten steel at a speed of 0.55 m / s for deoxidation and oxygen content (≤10 ppm) control. After the melt is cleared, samples are taken to test the content of each element in the molten steel.

[0081] 6) Turn on the vacuum pump, set the refining power to 100KW, observe the metal raw materials in the crucible through the observation hole until they are clear, continue refining until the molten steel begins to splash, then turn off the power to cool down and strengthen the stirring of the melt.

[0082] 7) Hoist the steel ingot mold and flow channel, turn on the vacuum pump, evacuate to <5Pa, supply 100KW power, let stand for 10 minutes before pouring, and then operate the handle to tilt the crucible for pouring.

[0083] 8) After casting, cool down, then open the venting valve to break the vacuum, open the furnace door, take out and disassemble the steel ingot mold, take out the hull structure steel ingot, and carry out billet forging and multi-pass rolling to obtain the steel ingot of the required specifications and dimensions.

[0084] 9) The steel ingot is held at 880℃ for 1.5h and then quenched. It is then heated to 700℃ for 1h and quenched again. Finally, it is tempered and heated to 540℃ for 1h and then air-cooled to room temperature. The resulting hull structure steel has a multi-type structure of ferrite and tempered lath martensite.

[0085] Example 3

[0086] 1. Ingredients

[0087] Accurately weigh 200 kg per furnace; the chemical composition of the high-strength and tough hull structural steel is designed as follows: C 0.052%, Si 0.038%, Mn 0.95%, Ni 4.5%, Cr 0.50%, Mo 0.45%, Al 0.025%, V 0.055%, Nb 0.05%, Ti 0.016%, P 0.0035%, S 0.002%, balance Fe.

[0088] 2. Smelting operation

[0089] 1) Add the raw materials in such a way that the larger pieces are placed near the inner wall of the crucible and the smaller pieces are placed near the center of the crucible;

[0090] 2) Close the furnace cover, turn on the vacuum pump, and evacuate the furnace to a vacuum level of <3.5 Pa;

[0091] 3) Set the power of the melting furnace to 90KW, and heat it up. When the metal in the crucible begins to melt, increase the power to 140KW to melt it.

[0092] 4) When the melting reaches the point of splashing, reduce the power to 80KW, turn off the vacuum pump, and at the same time, fill the furnace with argon gas to increase the vacuum pressure of the furnace body to 300Pa;

[0093] 5) The raw materials for the ship hull structure steel were added to a crucible for vacuum melting at a temperature of 1650℃. A cored composite wire (Mg 7.5%, calcium powder 5.0%, barium powder 5.0%, LaCe composite rare earth 4.5% (La to Ce mass ratio 1:1), Ti) was used. 0.2%, balance Fe; Mg wire diameter is 2.5mm; LaCe composite rare earth wire diameter is 3.0mm, oxygen content of LaCe composite rare earth wire <50ppm; Ti wire diameter is 2.0mm; calcium barium powder particle size is 1.5~2.0mm; iron sheet wall thickness is 0.20mm). Secondary materials are added to the crucible through the sealed wire feeding mechanism installed on the vacuum induction furnace cover. The feeding weight of the cored composite wire is 0.03% of the total weight of the raw materials. Based on the conversion between the feeding weight of the cored composite wire and the length of the composite wire, a composite wire with a diameter of 13mm and a length of 2m is fed into the molten steel at a speed of 0.3m / s for deoxidation and oxygen content (≤10ppm) control. After melting and clearing, samples are taken to test the content of each element in the molten steel.

[0094] 6) Turn on the vacuum pump, set the refining power to 110KW, observe the metal raw materials in the crucible through the observation hole until they are clear, continue refining until the molten steel begins to splash, then turn off the power to cool down and strengthen the stirring of the melt.

[0095] 7) Hoist the steel ingot mold and flow channel, turn on the vacuum pump, evacuate to <5Pa, then power on 80KW, let stand for 15 minutes before pouring, and then operate the handle to tilt the crucible for pouring.

[0096] 8) After casting, cool down, then open the venting valve to break the vacuum, open the furnace door, take out and disassemble the steel ingot mold, take out the hull structure steel ingot, and carry out billet forging and multi-pass rolling to obtain the steel ingot of the required specifications and dimensions.

[0097] 9) The steel ingot is quenched after being held at 900℃ for 1 hour, then heated to 660℃ for 1.5 hours and quenched again, and finally tempered and heated to 550℃ for 1.5 hours and air-cooled to room temperature. The resulting hull structural steel has a multi-type microstructure of ferrite, tempered sorbite and tempered lath martensite.

[0098] Comparative Example 1

[0099] Commercially available 980 steel for shipbuilding was used as a comparative example 1.

[0100] Characterization and performance testing

[0101] 1) Figure 3 Characterization diagram of the multi-component nanophase structure in the middle layer of the high-strength and high-toughness ship hull structural steel prepared for extreme cold environments in Example 1; by Figure 3 It is known that the multi-component nanophase in steel is a composite structure of particles. This is because the nano-sized carbonitrides, metal compounds and other particles formed during phase transformation, aging and other processes take the nano-sized oxides formed in the steel melt as the core, thereby achieving the hierarchical construction of the multi-component nanophase.

[0102] 2) Mechanical properties and low-temperature impact energy tests were conducted on the hull structural steel of Examples 1 to 3 and Comparative Example 1. The tensile method for mechanical property testing was carried out in accordance with GB / T228.1-2010 standard; the impact energy was carried out in accordance with GB / T 229-2007 standard. The results are shown in Table 1.

[0103] Table 1. Performance data of the hull structural steel prepared in Examples 1-3 and Comparative Example 1.

[0104]

[0105] As shown in Table 1, the mechanical properties and weld impact energy at -84℃ of the high-strength and high-toughness hull structural steel prepared in Examples 1-3 are significantly higher than those of the commercially available ship 980 steel in Comparative Example 1, while the yield strength ratio is significantly lower than that in Comparative Example 1. This indicates that the method can significantly improve the mechanical properties of hull structural steel and its weld impact energy at -84℃, and greatly reduce its yield strength ratio.

[0106] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing high-strength and high-toughness ship hull structural steel for extremely cold environments, characterized in that, Includes the following steps: The raw materials corresponding to the main chemical composition of high-strength and tough ship hull structural steel are mixed and vacuum melted. The resulting molten steel is fed into a cored composite wire and then refined and cast in sequence to obtain steel ingots. The steel ingot is forged, rolled and heat-treated in sequence to obtain high-strength and high-toughness ship hull structural steel for extremely cold environments; The main chemical composition of the high-strength and high-toughness ship hull structural steel includes: C 0.03~0.07%, Si 0.03~0.045%, Mn 0.80~1.10%, Ni 3.5~5.5%, Cr 0.35~0.55%, Mo 0.40~0.65%, Al 0.015~0.03%, V 0.03~0.08%, Nb 0.03~0.06%, Ti 0.005~0.02%, P≤0.01%, S≤0.003%, balance Fe; The chemical composition of the cored composite wire, by mass percentage, is: LaCe 2~5%, Mg 4~9%, Ti 0.1~0.3%, Ca 3~6%, Ba 3~6%, with the balance being Fe; the mass ratio of La to Ce is 1:

1. The heat treatment includes sequential quenching, secondary quenching, and tempering. The temperature of the first quenching is 850~920℃, and the holding time is 1~2h; the temperature of the second quenching is 650~720℃, and the holding time is 1~2h; the temperature of the tempering is 530~580℃, and the holding time is 1~2h.

2. The preparation method according to claim 1, characterized in that, The cored composite wire is made of iron sheet covered with a metal core material; the metal core material includes Mg wire, LaCe composite rare earth wire, Ti wire and calcium barium powder; the diameter of the cored composite wire is 8~13.5mm.

3. The preparation method according to claim 2, characterized in that, The diameter of the Mg wire is 1~3mm; the diameter of the LaCe composite rare earth wire is 1.5~3.5mm; the diameter of the Ti wire is 1~2.5mm; the particle size of the calcium barium powder is 0.1~2.5mm; and the wall thickness of the iron sheet is 0.1~0.2mm.

4. The preparation method according to claim 2 or 3, characterized in that, The oxygen content of the LaCe composite rare earth fiber is <50ppm.

5. The preparation method according to claim 1 or 2, characterized in that, The feeding speed of the core-coated composite line is 0.05~1.5m / s; the feeding weight of the core-coated composite line is 0.015~1.0% of the total weight of the raw materials.

6. The preparation method according to claim 1, characterized in that, The refining power is 80~160KW.

7. The high-strength and high-toughness ship hull structural steel for extremely cold environments prepared by the preparation method according to any one of claims 1 to 6, characterized in that, The matrix structure of the high-strength and high-toughness hull structural steel includes two or more of ferrite, tempered sorbite, and tempered martensite.

8. The application of the high-strength and high-toughness hull structural steel for extremely cold environments as described in claim 7 in ship equipment serving in extremely cold sea environments, wherein the temperature of the extremely cold sea environment is -50~-84℃.

Images