High-strength light-weight alloy structural steel resistant to extreme working conditions and method for manufacturing same

Abstract

This invention relates to a high-strength, weight-reducing alloy structural steel resistant to extreme working conditions and its preparation method, belonging to the field of alloy structural steel preparation technology. To address the problems of low strength under extreme working conditions and the inability to achieve high strength and weight reduction synergistically in existing alloy structural steels, this invention provides a high-strength, weight-reducing alloy structural steel resistant to extreme working conditions and its preparation method, including smelting, ingot casting, heating and rolling, cooling and annealing, and heat treatment processes. This invention, through precise composition design and process coupling control, prepares an alloy structural steel with both high strength and excellent ultra-low temperature toughness, with an impact energy ≥60J at -50℃ and ≥100J at -20℃. This invention achieves grain refinement and microstructure improvement through Nb microalloying and process optimization, balancing high strength and lightweight, improving equipment efficiency and reducing energy consumption. Key parameters throughout the process are controllable, product hardness, flaw detection, and surface precision meet standards, the process is compatible with existing production lines, and it can be mass-produced.

Description

Technical Field

[0001] This invention belongs to the field of alloy structural steel preparation technology, and particularly relates to a high-strength, weight-reducing alloy structural steel resistant to extreme working conditions and its preparation method. Background Technology

[0002] As a core structural material for high-end equipment, alloy structural steel's mechanical properties and lightweighting level directly determine the equipment's operational efficiency, structural safety, and service life. While existing alloy structural steels can meet technical standards under conventional operating conditions, they generally suffer from insufficient strength and reduced toughness under extreme conditions such as ultra-low temperatures and high loads, making it difficult to guarantee long-term stable service. Simultaneously, the industry has long faced the technical bottleneck of achieving high strength and weight reduction in tandem. Traditional methods of increasing strength often rely on high alloy content, high carbon content designs, or conventional microalloying processes. This not only increases material density, failing to meet lightweighting requirements, but also easily leads to problems such as brittle phase precipitation at grain boundaries and deterioration of banded structures, resulting in decreased material ductility, toughness, and weldability, further limiting its application in extreme conditions.

[0003] In existing technologies, some steel mills have attempted to improve the strength and toughness of alloy structural steel by adjusting heat treatment processes. However, due to a lack of precise composition design and process coupling control, it is difficult to simultaneously improve strength and low-temperature toughness, and weight reduction cannot be achieved. Conventional rolling and annealing processes have low control precision, and unreasonable matching of key parameters such as heating time, final rolling temperature, and annealing temperature can easily lead to coarse grains and uneven carbide precipitation, which not only affects the improvement of material strength but also makes it impossible to achieve reasonable control of material density through microstructure optimization. In addition, existing alloy structural steels are not adequately designed for ultra-low temperature impact, and the impact energy at -50℃ is difficult to reach above 60J, making them unsuitable for use in extreme ultra-low temperature conditions such as polar regions and deep seas. Summary of the Invention

[0004] To address the problems of low strength under extreme working conditions and the inability to achieve high strength and weight reduction in tandem with existing alloy structural steels, this invention provides a high-strength, weight-reducing alloy structural steel resistant to extreme working conditions and its preparation method.

[0005] The technical solution of the present invention:

[0006] A high-strength, weight-reducing alloy structural steel resistant to extreme working conditions, comprising the following chemical composition by weight percentage: C: 0.18~0.20wt%, Si: 1.50~1.60wt%, Mn: 1.00~1.30wt%, P≤0.020wt%, S≤0.010wt%, Cr: 0.80~1.00wt%, V: 1.20~1.30wt%, Mo: 0.20~0.30wt%, and Nb: 0.010~0.015wt%, with the remainder being Fe and unavoidable impurities.

[0007] A method for preparing a high-strength, weight-reducing alloy structural steel resistant to extreme working conditions includes the following steps:

[0008] Step 1: Smelting Process

[0009] The steel is smelted using an electric furnace or converter. During tapping, slag is controlled and alloys are added for pre-deoxidation and alloying. After tapping, LF refining is performed, with the refining time controlled to be no less than 60 minutes, including a white slag holding time of no less than 30 minutes. Then, VD vacuum degassing is performed, held at a vacuum degree ≤67Pa for at least 15 minutes. After degassing, the hydrogen content of the molten steel is ≤1.5ppm. The alloy element composition is controlled during the smelting process to finally obtain molten steel.

[0010] Step 2, Ingot Casting Process:

[0011] The molten steel, after VD vacuum degassing treatment, is then cast at a temperature controlled between 1548 and 1553°C; Step 3: Heating and rolling process:

[0012] The steel ingot is hot-sent and heated, and held at 1250~1280℃ for at least 8 hours; after heating, it is rolled, and the final rolling temperature is controlled at 900~950℃, and rolled into steel of the target size.

[0013] Step 4: Cooling and Annealing Process

[0014] The rolled steel is fed into a slow cooling pit to be slowly cooled to room temperature; then it undergoes spheroidizing annealing treatment to obtain annealed steel.

[0015] Step 5: Heat treatment process:

[0016] The obtained steel is subjected to quenching and tempering treatment. The quenching treatment is to heat to 920±10℃ and hold for oil quenching. The tempering treatment is a two-stage tempering process. The first tempering temperature is 260±10℃, and after holding, it is cooled to room temperature. Then, the second tempering is performed at a tempering temperature of 260±10℃ and held for a second time.

[0017] Furthermore, in the smelting process described in step one, the temperature of the molten steel is controlled at 1580~1630℃ when tapping, the carbon content in the molten steel is controlled at 0.05~0.09wt%, and the phosphorus content is ≤0.010wt%.

[0018] Furthermore, in the casting process described in step two, the casting process is divided into an ingot body casting stage and a cap casting stage. The casting time for the ingot body casting stage is 840~960s, and the casting volume is 45.8~52.4kg / s. The casting time for the cap casting stage is 360~480s, and the casting volume is 12.5~16.7kg / s. After casting, a steel ingot is obtained.

[0019] Furthermore, in the heating and rolling process described in step three, the heating process includes: holding at a low temperature of less than 900°C for 15 minutes, and then heating up to 1250-1280°C at a heating rate of 30-110°C / h.

[0020] Furthermore, in the heating and rolling process described in step three, the rolling process adopts a large reduction process, with a reduction of not less than 50mm per pass; after rolling, 80~120mm of the ingot tail is removed, and 50~100mm below the riser is removed.

[0021] Furthermore, in the cooling and annealing process described in step four, the temperature upon entering the pit is controlled at 400~600℃.

[0022] Furthermore, in the cooling and annealing process described in step four, the spheroidizing annealing treatment involves heating the steel to 680~720℃, holding it at that temperature for 3 hours, and then cooling it to room temperature in the furnace.

[0023] Furthermore, in the heat treatment process described in step five, the holding time for quenching is 1 hour.

[0024] Furthermore, in step five, the holding time for both the first and second tempering processes is 2 hours.

[0025] The beneficial effects of this invention are:

[0026] This invention, through precise chemical composition design and process coupling control, enables alloy structural steel to possess excellent ultra-low temperature toughness and high strength, achieving ultra-low temperature impact performance of ≥60J at -50℃ and ≥100J at -20℃. It is fully adaptable to the application requirements of ultra-low temperature extreme working conditions such as polar regions and deep seas. At the same time, after undergoing the optimal heat treatment process of quenching at 920℃ and tempering twice at 260℃, the material strength is significantly improved, and it can still maintain stable mechanical properties under high load extreme working conditions, completely solving the core problems of low strength and toughness reduction of existing materials under extreme working conditions.

[0027] This invention employs precise composition and process design, such as Nb element microalloying to refine grains and extending heating time to improve banded structure, to enhance the mechanical properties of materials while effectively controlling material density, thereby achieving lightweighting and weight reduction. This breaks through the technical bottleneck in the industry where high strength and weight reduction are difficult to achieve simultaneously. The alloy structural steel prepared using this invention can be applied to high-end equipment, directly improving equipment operating efficiency and reducing energy consumption during equipment service.

[0028] This invention, through precise control of key process parameters throughout the smelting, rolling, and annealing processes, determines 700℃ as the optimal annealing temperature. Combined with furnace cooling, this allows the annealed hardness of the material to be controlled between 189 and 212 HB, far below the technical standard requirement of ≤230 HB. Furthermore, the microstructure after annealing consists of ferrite and pearlite, with a large number of fine and dispersed carbides precipitating in the matrix, effectively avoiding problems such as coarse grains, uneven carbide precipitation, and precipitation of brittle phases at grain boundaries. At the same time, the banded structure is significantly improved, and the material's ductility, toughness, and weldability are greatly enhanced. This solves the problem of decreased material processing performance caused by traditional processes and expands the processing and application scenarios of the material.

[0029] This invention clarifies the key process parameters and control standards for the entire production process, including heating temperature of 1250~1280℃, high-temperature holding time of ≥8h, final rolling temperature of 900~950℃, and casting speed parameters for the ingot body and cap during smelting and casting. Furthermore, the internal flaw detection quality level of the steel meets the GB / T4162 Class A flaw detection requirements, and the surface accuracy is strictly controlled to 0.2mm. The process parameters are clear and highly controllable, effectively ensuring the consistency and stability of product quality. Simultaneously, the entire preparation process is implemented based on conventional electric / converter smelting and rolling equipment, requiring no additional specialized equipment, making it easily compatible with existing industrial production systems and feasible for large-scale production. Attached Figure Description

[0030] Figure 1 Metallographic images of the hot-rolled state samples of Example 1 and the annealed state samples obtained at different annealing temperatures of Example 1, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0032] Example 1

[0033] This embodiment provides a high-strength, weight-reducing alloy structural steel resistant to extreme working conditions and its preparation method.

[0034] The chemical composition of the alloy structural steel in this embodiment, by weight percentage, includes: C: 0.20wt%, Si: 1.60wt%, Mn: 1.30wt%, P: 0.016wt%, S: 0.008wt%, Cr: 0.80wt%, V: 1.30wt%, Mo: 0.30wt%, and Nb: 0.015wt%, with the remainder being Fe and unavoidable impurities.

[0035] The preparation method of alloy structural steel in this embodiment has the following process flow: electric / converter smelting + LF refining + VD vacuum refining → ingot casting → hot delivery to the steel rolling mill for bar rolling → slow cooling → annealing → straightening → automatic line flaw detection → inspection and testing → delivery.

[0036] Step 1: Smelting Process

[0037] This embodiment adopts a three-stage smelting process of electric / converter smelting + LF refining + VD vacuum refining. The specific process and parameter control are as follows:

[0038] (1) Electric / converter smelting:

[0039] Qualified scrap steel and molten iron are selected as raw materials and added to the furnace for melting and preliminary oxidation, decarburization, and dephosphorization. When the molten steel reaches 1606℃, it is tapped. Slag discharge is strictly controlled during tapping, and alloys are added for preliminary pre-deoxidation and alloying. The carbon content in the molten steel is controlled at 0.06wt% and the phosphorus content at 0.003wt% at tapping, creating favorable conditions for subsequent refining.

[0040] (2) LF Refining:

[0041] After the molten steel is transferred to the LF station, electrode heating is activated and white slag refining is carried out. The entire refining process is held for 83 minutes, of which the white slag is held for 55 minutes. The core purpose of this process is to deeply desulfurize, remove inclusions from the steel, and precisely fine-tune the chemical composition to ultimately stabilize it within the target range.

[0042] (3) VD vacuum degassing:

[0043] The LF-refined steel ladle is hoisted to the VD station for vacuum treatment. It is held at a vacuum level ≤67 Pa for ≥15 minutes to thoroughly remove harmful gases such as hydrogen, oxygen, and nitrogen from the molten steel. After vacuuming, the molten steel undergoes hydrogen content testing, requiring a hydrogen content of 0.91 ppm to ensure high purity of the final material and avoid metallurgical defects such as white spots. This is a crucial prerequisite for ensuring low-temperature impact toughness.

[0044] During the smelting process, the amount of microalloying element Nb is precisely controlled. Nb is used to refine the grains and form dispersed carbonitrides, replacing traditional high-density alloying elements. This reduces the overall density of the material while improving strength, thus achieving weight reduction. During the refining process, the purity of the molten steel is strictly controlled to remove harmful impurities and prevent the precipitation of brittle phases at grain boundaries, ensuring the material's low-temperature toughness.

[0045] Step 2, Ingot Casting Process:

[0046] This embodiment employs a die casting process. Molten steel is immediately poured after VD vacuum degassing treatment. Before casting, the tundish and ingot mold are ensured to be dry and rust-free. The casting temperature is strictly controlled within a narrow range of 1550℃.

[0047] The casting process is divided into two stages: ingot body casting and cap casting, each employing different casting speed control strategies.

[0048] (1) Casting stage of ingot body: casting time 840s, casting volume 52.4kg / s. In this stage, the steel ingot mold body is filled quickly and smoothly to avoid the molten steel staying in the mold for too long, which would cause excessive temperature drop, thereby preventing surface defects such as "double skin" and "cold shut", and ensuring that the outer shell of the steel ingot is solidified uniformly.

[0049] (2) Capping and clamping: Pouring time 360s, pouring rate 16.7kg / s. When the molten steel fills the ingot body and begins to enter the top heat-insulating cap (cap opening), the pouring speed needs to be significantly reduced. The purpose is to keep the molten steel in the cap opening in a liquid state in the later stage of solidification, continuously feeding the ingot body below, thereby controlling the shrinkage cavity at the cap opening and ensuring the compactness of the ingot body.

[0050] Step 3: Heating and rolling process:

[0051] After demolding, the steel ingot is hot-transferred to the heating furnace of the rolling mill. The temperature in the low-temperature section is controlled below 900℃ for 15 minutes to prevent thermal stress cracking caused by a sudden temperature rise. The heating section raises the temperature to 1280℃ at a rate of 50℃ / h. The high-temperature holding section holds the temperature at 1280℃ for 8 hours to ensure uniform temperature inside and outside the ingot and to allow alloying elements to fully dissolve, especially carbide-forming elements such as V and Mo, laying the foundation for subsequent rolling and microstructure control.

[0052] The final rolling temperature is set at 950℃ to ensure good plasticity of the steel while preventing grain coarsening due to excessive temperature. A large bar mill is used for rolling. A large reduction process is implemented during rolling, with a reduction of 60mm per pass, to effectively break up the as-cast structure and improve core density. The steel billet is finally rolled into round bars of the target size, φ92~92.5mm. After rolling, defective portions of the ingot are removed according to regulations: 120mm is removed from the ingot tail, and 100mm is removed below the riser, to ensure that the head and tail of the finished product are free of shrinkage cavities, porosity, and other defects.

[0053] Step 4: Cooling and Annealing Process

[0054] (1) Controlled cooling after rolling:

[0055] The rolled steel is heated to a high temperature and requires controlled cooling to prevent abnormal microstructure or cracks. The steel is immediately placed in a slow cooling pit, with the initial temperature controlled at 500°C. It is then slowly cooled to room temperature within the pit to prevent hydrogen-induced cracking.

[0056] (2) Annealing treatment:

[0057] To reduce the hardness of steel, improve its machinability, and prepare the microstructure for final heat treatment, spheroidizing annealing is necessary. Through process testing, this embodiment determined the optimal annealing regime as follows: heating the steel to 700℃, holding for 3 hours, and then furnace cooling to room temperature. After this process, the annealed hardness of the steel is uniformly distributed between 188 and 212 HBW, fully meeting the standard requirement of ≤230 HBW. Metallographic analysis shows that annealing at 700℃ yields an ideal ferrite matrix with finely dispersed carbides, ensuring both low hardness and providing a good carbon concentration gradient for subsequent quenching.

[0058] Annealed steel may undergo some bending deformation during the cooling process, requiring straightening using a pressure straightener or roller straightener to ensure its straightness meets delivery requirements. Subsequently, the steel undergoes full-length non-destructive testing using automated ultrasonic flaw detection equipment. The internal flaw detection quality level meets the GB / T4162 Class A flaw detection requirements, ensuring the absence of fatal defects such as cracks and slag inclusions. The surface finish of the steel is controlled to within 0.2mm.

[0059] Step 5: Heat treatment process:

[0060] In order to obtain the target mechanical properties, the finished steel is subjected to quenching and low-temperature tempering treatment in this embodiment.

[0061] (1) Quenching treatment:

[0062] The steel is heated to 920°C and held for 1 hour based on the effective thickness to ensure complete austenitization and full dissolution of carbides. It is then oil-quenched to obtain a martensitic structure.

[0063] (2) Tempering treatment:

[0064] Immediately after quenching, tempering is performed using a two-stage tempering process. The first tempering temperature is 260℃, and the holding time is 2 hours. The steel is then cooled to room temperature, followed by a second tempering at 260℃ for 2 hours. The purpose of this two-stage tempering is to fully eliminate quenching stress, stabilize the microstructure, and promote the transformation of retained austenite, ultimately obtaining tempered martensite. This ensures both ultra-high strength and excellent low-temperature impact toughness.

[0065] Example 2

[0066] This embodiment provides a high-strength, weight-reducing alloy structural steel resistant to extreme working conditions and its preparation method.

[0067] The chemical composition of the alloy structural steel in this embodiment, by weight percentage, includes: C: 0.18wt%, Si: 1.50wt%, Mn: 1.00wt%, P: 0.015wt%, S: 0.008wt%, Cr: 0.80wt%, V: 1.20wt%, Mo: 0.20wt%, and Nb: 0.010wt%, with the remainder being Fe and unavoidable impurities.

[0068] The preparation method of alloy structural steel in this embodiment has the following process flow: electric / converter smelting + LF refining + VD vacuum refining → ingot casting → hot delivery to the steel rolling mill for bar rolling → slow cooling → annealing → straightening → automatic line flaw detection → inspection and testing → delivery.

[0069] Step 1: Smelting Process

[0070] This embodiment adopts a three-stage smelting process of electric / converter smelting + LF refining + VD vacuum refining. The specific process and parameter control are as follows:

[0071] (1) Electric / converter smelting:

[0072] Qualified scrap steel and molten iron are selected as raw materials and added to the furnace for melting and preliminary oxidation, decarburization, and dephosphorization. When the molten steel temperature reaches 1614℃, it is tapped. Slag discharge is strictly controlled during tapping, and alloys are added for preliminary pre-deoxidation and alloying. The carbon content in the molten steel is controlled at 0.08wt% and the phosphorus content at 0.004wt% at tapping, creating favorable conditions for subsequent refining.

[0073] (2) LF Refining:

[0074] After the molten steel is transferred to the LF station, electrode heating is activated and white slag refining is carried out. The entire refining process is held for 65 minutes, of which the white slag is held for 45 minutes. The core purpose of this process is to deeply desulfurize, remove inclusions from the steel, and precisely fine-tune the chemical composition to ultimately stabilize it within the target range.

[0075] (3) VD vacuum degassing:

[0076] The LF-refined steel ladle is hoisted to the VD station for vacuum treatment. It is held at a vacuum level ≤67 Pa for ≥15 minutes to thoroughly remove harmful gases such as hydrogen, oxygen, and nitrogen from the molten steel. After vacuuming, the molten steel undergoes hydrogen content testing, requiring a hydrogen content of 1.05 ppm to ensure high purity of the final material and avoid metallurgical defects such as white spots. This is a crucial prerequisite for ensuring low-temperature impact toughness.

[0077] During the smelting process, the amount of microalloying element Nb is precisely controlled. Nb is used to refine the grains and form dispersed carbonitrides, replacing traditional high-density alloying elements. This reduces the overall density of the material while improving strength, thus achieving weight reduction. During the refining process, the purity of the molten steel is strictly controlled to remove harmful impurities and prevent the precipitation of brittle phases at grain boundaries, ensuring the material's low-temperature toughness.

[0078] Step 2, Ingot Casting Process:

[0079] This embodiment employs a die casting process. Molten steel is immediately poured after VD vacuum degassing treatment. Before casting, the tundish and ingot mold are ensured to be dry and rust-free. The casting temperature is strictly controlled within a narrow range of 1550℃.

[0080] The casting process is divided into two stages: ingot body casting and cap casting, each employing different casting speed control strategies.

[0081] (1) Casting stage of ingot body: casting time 960s, casting volume 45.8kg / s. In this stage, the steel ingot mold body is filled quickly and smoothly to avoid the molten steel staying in the mold for too long, which would cause excessive temperature drop, thereby preventing surface defects such as "double skin" and "cold shut", and ensuring that the outer shell of the steel ingot is solidified uniformly.

[0082] (2) Capping and clamping: Pouring time 480s, pouring rate 12.5kg / s. When the molten steel fills the ingot body and begins to enter the top heat-insulating cap (capping), the pouring speed needs to be significantly reduced. The purpose is to keep the molten steel in the capping in the late solidification stage, continuously feeding the ingot body below, thereby controlling the shrinkage cavity at the capping part and ensuring the compactness of the ingot body.

[0083] Step 3: Heating and rolling process:

[0084] After demolding, the steel ingot is hot-transferred to the heating furnace of the rolling mill. The temperature in the low-temperature section is controlled below 900℃ for 15 minutes to prevent thermal stress cracking caused by a sudden temperature rise. The heating section raises the temperature to 1250℃ at a rate of 60℃ / h. The high-temperature holding section holds the temperature at 1250℃ for 8 hours to ensure uniform temperature inside and outside the ingot and to allow alloying elements to fully dissolve, especially carbide-forming elements such as V and Mo, laying the foundation for subsequent rolling and microstructure control.

[0085] The final rolling temperature is set at 900℃ to ensure good plasticity of the steel while preventing grain coarsening due to excessive temperature. A large bar mill is used for rolling. A large reduction process is implemented during rolling, with a reduction of 50mm per pass, to effectively break up the as-cast structure and improve core density. The steel billet is finally rolled into round bars of the target size, φ92~92.5mm. After rolling, defective portions of the ingot are removed according to regulations: 80mm is removed from the ingot tail, and 50mm is removed below the riser, to ensure that the head and tail of the finished product are free of shrinkage cavities, porosity, and other defects.

[0086] Step 4: Cooling and Annealing Process

[0087] (1) Controlled cooling after rolling:

[0088] The rolled steel is heated to a high temperature and requires controlled cooling to prevent abnormal structures or cracks. The steel is immediately placed in a slow cooling pit, with the initial temperature controlled at 600°C. It is then slowly cooled to room temperature within the pit to prevent hydrogen-induced cracking.

[0089] (2) Annealing treatment:

[0090] To reduce the hardness of steel, improve its machinability, and prepare its microstructure for final heat treatment, spheroidizing annealing is necessary. Through process testing, this embodiment determines the optimal annealing regime as follows: heating the steel to 700°C, holding at that temperature for 3 hours, and then cooling it to room temperature in the furnace.

[0091] Annealed steel may undergo some bending deformation during the cooling process, requiring straightening using a pressure straightener or roller straightener to ensure its straightness meets delivery requirements. Subsequently, the steel undergoes full-length non-destructive testing using automated ultrasonic flaw detection equipment. The internal flaw detection quality level meets the GB / T4162 Class A flaw detection requirements, ensuring the absence of fatal defects such as cracks and slag inclusions. The surface finish of the steel is controlled to within 0.2mm.

[0092] Step 5: Heat treatment process:

[0093] In order to obtain the target mechanical properties, the finished steel is subjected to quenching and low-temperature tempering treatment in this embodiment.

[0094] (1) Quenching treatment:

[0095] The steel is heated to 920°C and held for 1 hour based on the effective thickness to ensure complete austenitization and full dissolution of carbides. It is then oil-quenched to obtain a martensitic structure.

[0096] (2) Tempering treatment:

[0097] Immediately after quenching, tempering is performed using a two-stage tempering process. The first tempering temperature is 260℃, and the holding time is 2 hours. The steel is then cooled to room temperature, followed by a second tempering at 260℃ for 2 hours. The purpose of this two-stage tempering is to fully eliminate quenching stress, stabilize the microstructure, and promote the transformation of retained austenite, ultimately obtaining tempered martensite. This ensures both ultra-high strength and excellent low-temperature impact toughness.

[0098] Comparative Example 1

[0099] The only difference between this comparative example and Example 1 is that the annealing temperature in step four, cooling and annealing, of this comparative example is 680°C.

[0100] Comparative Example 2

[0101] The only difference between this comparative example and Example 1 is that the annealing temperature in step four, cooling and annealing, of this comparative example is 720°C.

[0102] Comparative Example 3

[0103] The only difference between this comparative example and Example 1 is that step five, the heat treatment process, is different in this comparative example.

[0104] (1) Quenching treatment:

[0105] The steel is heated to 915℃ and held for 1.5 hours, then oil quenched.

[0106] (2) Tempering treatment:

[0107] Tempering is performed immediately after quenching, using two tempering processes. The first tempering temperature is 260℃ and the holding time is 2 hours. Then, the steel is cooled to room temperature and tempered a second time at 260℃ for 2 hours.

[0108] Figure 1 These are metallographic photographs of the hot-rolled samples of Example 1 and the annealed samples obtained at different annealing temperatures of Examples 1, 1 Comparative, and 2. Figure 1As shown, the microstructure of the hot-rolled state consists of ferrite, martensite, and bainite; after annealing at 680℃, the microstructure consists of ferrite and pearlite, with larger grains than the hot-rolled state. A small amount of carbides precipitate in the ferrite, and the amount of pearlite is slightly greater than that of ferrite; after annealing at 700℃, the microstructure consists of ferrite and pearlite, with larger grains than the hot-rolled state. The grain size is similar to that of the 680℃ annealed state, and the ferrite microstructure is significantly dominant in quantity. A large number of fine and dispersed carbides precipitate in the ferrite matrix; after annealing at 720℃, the microstructure consists of ferrite and pearlite, with larger grains than the hot-rolled state. However, compared with the 680℃ and 700℃ annealed states, the grains are finer, the amount of pearlite is significantly greater than that of ferrite, and the size of the carbides is significantly larger than that of the 680℃ and 700℃ annealed states.

[0109] Table 1 shows the hardness of the hot-rolled state and the hardness after annealing at different temperatures. The results show that the hardness value is 188-197 HBW when annealed at 700℃, and the hardness is higher when annealed at 680℃ and 720℃.

[0110] Table 1

[0111]

[0112] The mechanical properties of the alloy structural steels prepared in Examples 1, 2, and 3 were tested. Tensile properties were tested: standard tensile specimens were processed according to GB / T 228.1-2021, and four sets of parallel specimens were taken under the same process. The yield strength (Rp0.2), tensile strength (Rm), elongation after fracture (A%), and reduction of area (Z%) were tested using a universal testing machine. Impact properties were tested: Charpy U-notch (KU2) specimens were processed according to GB / T 229-2020, and three sets of parallel specimens were taken under the same process. The impact absorption energy (KU2) was tested at 20℃ and -50℃ on a low-temperature impact testing machine. The results are shown in Table 2.

[0113] Table 2

[0114]

[0115] The tensile properties (Rp0.2, Rm, A%, Z%) and impact properties (20℃ / -50℃ KU2) in Table 2 are the test results of multiple parallel specimens under the same heat treatment regime.

[0116] Tensile data (Rp0.2 / Rm / A% / Z%) show that for each specimen, all four mechanical property indicators meet the standard requirements of ≥1200MPa (Rp0.2), ≥1300MPa (Rm), ≥11% (A%), and ≥50% (Z%). Furthermore, the data within each group exhibits minimal fluctuation, demonstrating a balance of strength and plasticity, along with controllable processing. Impact data (20℃ / -50℃ KU2) shows that each specimen corresponds to one impact energy value. The three sets of values ​​in the table represent independent test results for three parallel specimens under the same process. In Examples 1 and 2, under the two-stage process of quenching at 920℃ and tempering at 260℃, the impact energy at 20℃ is ≥100J, and at -50℃ it is ≥60J, fully meeting the standards and far exceeding the minimum requirements. This demonstrates excellent and stable ultra-low temperature toughness, effectively ensuring consistent quality in large-scale product production.

Claims

1. A high-strength, weight-reducing alloy structural steel resistant to extreme working conditions, characterized in that, The chemical composition by weight percentage includes: C: 0.18~0.20wt%, Si: 1.50~1.60wt%, Mn: 1.00~1.30wt%, P≤0.020wt%, S≤0.010wt%, Cr: 0.80~1.00wt%, V: 1.20~1.30wt%, Mo: 0.20~0.30wt%, and Nb: 0.010~0.015wt%, with the remainder being Fe and unavoidable impurities.

2. A method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions as described in claim 1, characterized in that, Includes the following steps: Step 1: Smelting Process The steel is smelted using an electric furnace or converter. During tapping, slag is controlled and alloys are added for pre-deoxidation and alloying. After tapping, LF refining is performed, with the refining time controlled to be no less than 60 minutes, including a white slag holding time of no less than 30 minutes. Then, VD vacuum degassing is performed, held at a vacuum degree ≤67Pa for at least 15 minutes. After degassing, the hydrogen content of the molten steel is ≤1.5ppm. The alloy element composition is controlled during the smelting process to finally obtain molten steel. Step 2, Ingot Casting Process: The molten steel, after VD vacuum degassing treatment, is then cast at a temperature controlled between 1548 and 1553°C; Step 3: Heating and rolling process: The steel ingot is hot-sent and heated, and held at 1250~1280℃ for at least 8 hours; after heating, it is rolled, and the final rolling temperature is controlled at 900~950℃, and rolled into steel of the target size. Step 4: Cooling and Annealing Process The rolled steel is fed into a slow cooling pit to be slowly cooled to room temperature; then it undergoes spheroidizing annealing treatment to obtain annealed steel. Step 5: Heat treatment process: The obtained steel is subjected to quenching and tempering treatment. The quenching treatment is to heat to 920±10℃ and hold for oil quenching. The tempering treatment is a two-stage tempering process. The first tempering temperature is 260±10℃, and after holding, it is cooled to room temperature. Then, the second tempering is performed at a tempering temperature of 260±10℃ and held for a second time.

3. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 2, characterized in that, In the smelting process described in step one, the temperature of the molten steel is controlled at 1580~1630℃ when tapping, the carbon content in the molten steel is controlled at 0.05~0.09wt%, and the phosphorus content is ≤0.010wt%.

4. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 2 or 3, characterized in that, In the casting process described in step two, the casting process is divided into the ingot body casting stage and the cap casting stage. The casting time of the ingot body casting stage is 840~960s, and the casting volume is 45.8~52.4kg / s. The casting time of the cap casting stage is 360~480s, and the casting volume is 12.5~16.7kg / s. After casting, a steel ingot is obtained.

5. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 4, characterized in that, In the heating and rolling process described in step three, the heating process includes: holding at a low temperature of less than 900°C for 15 minutes, and then heating up to 1250-1280°C at a heating rate of 30-110°C / h.

6. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 5, characterized in that, In the heating and rolling process described in step three, the rolling process adopts a large reduction process, with a reduction of not less than 50mm per pass; after rolling, 80~120mm of the ingot tail is removed, and 50~100mm below the riser is removed.

7. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 6, characterized in that, In the cooling and annealing process described in step four, the temperature of the pit is controlled at 400~600℃.

8. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 7, characterized in that, In the cooling and annealing process described in step four, the spheroidizing annealing treatment involves heating the steel to 680~720℃, holding it at that temperature for 3 hours, and then cooling it to room temperature in the furnace.

9. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 8, characterized in that, In the heat treatment process described in step five, the holding time for quenching is 1 hour.

10. The method for preparing high-strength, weight-reducing alloy structural steel resistant to extreme working conditions according to claim 9, characterized in that, In step five, the holding time for both the first and second tempering processes is 2 hours.

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