Engineering steel for extreme conditions and short process production method thereof

Abstract

This invention relates to the field of steel material manufacturing, addressing the problem that existing steels struggle to simultaneously ensure weather resistance and low-temperature toughness. It provides an engineering steel for extreme conditions and its short-process preparation method, comprising, by weight percentage: C 0.10%-0.15%, Si 0.17%-0.37%, Mn 1.60%-2.00%, Ni 2.5%-5.0%, Cu 0.35%-0.55%, Cr 0.35%-0.55%, V 0.13%-0.15%, and Nb 0.01-0.02%. This invention achieves weather resistance by eliminating high phosphorus (P) and adopting a Cu-Cr-high Ni composition, thus eliminating the harmful effects of P on temperature and toughness from the compositional source. It shifts from existing alloying compromise strategies to a method of compositional purification and alloy optimization, no longer relying on the harmful element P, but achieving steel performance suitable for extreme conditions through purification and a better alloy combination; simultaneously, the high Ni content also ensures high low-temperature impact toughness. This invention employs low-carbon Mn solid solution strengthening, V / Nb composite microalloying for fine grain strengthening, and precipitation strengthening to ensure higher strength in steel. The engineering steel provided by this invention not only has high strength and toughness but is also suitable for freeze-thaw environments as well as rainy environments.

Description

Technical Field

[0001] This invention relates to the field of steel material manufacturing, and more specifically, to an engineering steel for use under extreme conditions and a short-process preparation method thereof. Background Technology

[0002] When steel is used in extreme environments, the performance requirements are far higher than those for conventional steel. Extreme environments refer to conditions involving prolonged low-temperature operation and short-term freeze-thaw cycles, such as the high-altitude freeze-thaw environment of the Qinghai-Tibet Plateau and the rainy summer environment of southern Tibet. In these areas, transportation conditions and operating environments are harsh, necessitating firstly, higher strength grades to extend service life. Secondly, due to freeze-thaw conditions, the ductile-brittle transition temperature of the steel needs to be lowered, and its low-temperature impact toughness improved. Furthermore, the large temperature differences in freeze-thaw environments require the steel to have good repeated bending resistance. Finally, to cope with the humid weather of the rainy season, the steel needs good weather resistance.

[0003] Existing technologies typically employ high carbon (C) designs to enhance strength, Cu-P alloying to ensure weather resistance, and the addition of 1.5-3.5% nickel to improve impact toughness at -30°C. The Cu-P alloying approach requires a phosphorus (P) content of approximately 0.15%, but to ensure low-temperature toughness, the P content must not exceed 0.015%. Therefore, the Cu-P-Ni system struggles to simultaneously guarantee both weather resistance and low-temperature toughness. Summary of the Invention

[0004] The purpose of this invention is to provide an engineering steel for use under extreme conditions and its short-process preparation method, which solves the problem that existing steels cannot simultaneously ensure weather resistance and low-temperature toughness. The invention achieves the triple goals of high strength, high and low temperature impact resistance, and weather resistance through a Cu-Cr-high Ni-high Mn-low C system.

[0005] The embodiments of the present invention are achieved through the following technical solutions:

[0006] An engineering steel for extreme conditions comprises, by weight percentage: C 0.10%-0.15%, Si 0.17%-0.37%, Mn 1.60%-2.00%, Ni 2.5%-5.0%, Cu 0.35%-0.55%, Cr 0.35%-0.55%, V 0.13%-0.15%, and Nb 0.01-0.02%.

[0007] Due to the large temperature difference under extreme conditions, reducing the coefficient of linear expansion of the material is not a viable approach. This invention directly employs a V and Nb composite microalloying method to ensure both forward and reverse bending performance. To ensure Cr forms an oxide film and avoid Cr deposition... X C YTo reduce toughness, this invention employs a low-C design, thereby increasing the Mn content to compensate for strength. At the same time, the composite microalloying of V and Nb ensures that both yield strength and tensile strength are improved.

[0008] Preferably, by weight percentage, the P content is ≤0.015%, the S content is ≤0.015%, the O content is ≤30ppm, and the N content is ≤60ppm.

[0009] The content of harmful elements such as Sn and As is ≤0.03%.

[0010] Preferably, the ferrite grain size is 8.0-9.5; the yield strength is ≥600MPa; the tensile strength is ≥700MPa; and the impact energy at -60℃ is ≥27J.

[0011] A short-process preparation method for the aforementioned engineering steel includes the following steps:

[0012] After preheating S100 scrap steel, it is fed into an electric arc furnace, melted, and then lime and magnesium carbon balls are added to form slag for smelting to obtain primary refined molten steel.

[0013] S200, under the CaO-Al2O3-SiO2 ternary low-basicity slag system, primary refined molten steel is heated and refined in an LF furnace to obtain secondary refined molten steel;

[0014] S300, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the secondary refined molten steel is degassed and refined in a VD furnace under vacuum, and then fed into a pure calcium wire to obtain tertiary refined molten steel.

[0015] S400 steel, after being refined three times, is processed through continuous casting, heating, and rolling processes to obtain engineering steel.

[0016] The raw material ratio of the CaO-Al2O3-SiO2 ternary low-alkalinity slag system is: 3-4 kg / t lime, 2.5-3.5 kg / t bauxite, 0.2-0.3 kg / t aluminum granules, and 0.2-0.3 kg / t silicon carbide.

[0017] Step S100 can oxidize and dephosphorize, remove carbon, and remove some impurities.

[0018] Step S200 can perform deep desulfurization, deoxygenation, and adsorption of impurities.

[0019] Step S300 reduces the risk of hydrogen embrittlement; calcium treatment modifies Al2O3 inclusions into low-melting-point calcium aluminate, improving castability.

[0020] Preferably, in step S100, the preheating temperature is 800-1000℃, the smelting temperature is 1620-1650℃, the smelting time is 30-40min, the final carbon content is 0.06wt%-0.08wt%, and the oxygen supply during smelting is 25-30Nm³ / t steel.

[0021] The purpose of preheating is at least to: remove impurities such as oil, paint, and zinc layers; reduce gases and inclusions in the steel; and reduce energy consumption of the electric arc furnace and improve melting efficiency. Therefore, the preheating time is related to the impurity content on the surface of the raw material, scrap steel, and thus this invention does not specify a particular preheating time.

[0022] Controlling the carbon content at the endpoint provides room for adjusting the composition during subsequent refining.

[0023] Preferably, in step S100, when refining and tapping steel, when the steel reaches 20vt%-30vt%, 1.5-2.0 kg / t of aluminum ferrooxidant is added for pre-deoxidation based on the lowest internally controlled composition value, followed by the addition of ferrosilicon and ferromanganese, and finally 3-4 kg / t of lime, which is added when the steel reaches 70vt%-80vt%. The tapping process is accompanied by bottom blowing argon and stirring.

[0024] The internal control component standards are as follows: by weight percentage, including: C 0.10%-0.15%, Si 0.17%-0.37%, Mn 1.60%-2.00%, Ni 2.5%-5.0%, Cu 0.35%-0.55%, Cr 0.35%-0.55%, V 0.13%-0.15%, Nb 0.01-0.02%, P content ≤0.015%, S content ≤0.015%, O content ≤30ppm, N content ≤60ppm; the content of harmful elements such as Sn and As ≤0.03%.

[0025] Preferably, in step S200, the ladle inlet temperature is ≥1560℃, the refining time is 40-50min, the basicity is maintained at 3-4, the white slag retention time is ≥15min, and ferrosilicon, low-carbon ferromanganese, ferronickel, copper blocks and ferrochrome alloy are added during refining to finely adjust the composition to the internal control target value.

[0026] Maintaining an alkalinity of 3-4 indicates that the CaO content in the protective slag is 3 to 4 times that of SiO2.

[0027] White slag refers to a white or grayish-white alkaline reducing slag formed during the ladle refining stage of steelmaking after a series of deoxidation and desulfurization operations. It has a high calcium oxide content and extremely low levels of unstable oxides such as iron oxide and manganese oxide. White slag formation is a core task of LF furnace refining, its fundamental purpose being to deeply purify molten steel under a reducing atmosphere. A white slag holding time of ≥15 minutes ensures sufficient duration for its deep purification effect. This prolonged white slag refining process guarantees sufficient time for the desulfurization reaction to proceed and for inclusions to float and be adsorbed by the slag, which is crucial for achieving high purity.

[0028] Preferably, in step S300, the ladle inlet temperature is 1630-1650℃, the refining time is 20-30 min, the vacuuming period is ≤6 min, and the high vacuum holding time is ≥20 min; after cavitation, calcium wire is fed, the pure calcium feed rate is 150-180 m / t, the soft blowing time after cavitation is ≥15 min, the argon flow rate is 30-50 NL / min, and the ladle outlet temperature is 1590-1610℃.

[0029] The ladle free space should be ≥900mm. Ladle free space, also known as ladle clearance, refers to the vertical distance between the molten steel surface and the top edge of the ladle or the lower surface of the ladle cover. It represents the top space within the ladle not occupied by molten steel. The minimum free space serves several purposes, including preventing molten steel splashing, such as: in continuous casting using the ladle → long nozzle → tundish protective pouring method, a larger free space allows for a relatively higher ladle position, providing the necessary physical installation and adjustment height for the long nozzle and its protective system. Insufficient space leads to installation difficulties, poor sealing, and the ingestion of air, which can generate new oxide inclusions such as Al2O3, severely affecting the cleanliness of the molten steel.

[0030] The vacuuming period of ≤6min refers to the time elapsed from the start of the VD vacuum pump to the point where the system pressure reaches the target high vacuum value, typically ≤67Pa. The faster the vacuuming speed, the sooner the molten steel begins to boil violently, resulting in higher degassing efficiency. If the vacuuming process is slow, the residence time at a medium vacuum level is long, and the molten steel risks absorbing nitrogen from the atmosphere, which is fatal for steel grades requiring low nitrogen content.

[0031] A high vacuum holding time of ≥20 min is necessary to ensure sufficient physicochemical reaction time. Hydrogen diffusion and flotation removal require time. For large-section, high-requirement marine steels, to prevent white spot defects, the hydrogen content must be reduced to extremely low levels, such as ≤1.5 ppm, necessitating a sufficient high vacuum holding time. Although denitrification is more difficult than dehydrogenation, it is still effective under high vacuum and strong stirring; the longer the holding time, the better the denitrification effect. Under high vacuum, C and O in the molten steel react to generate CO bubbles that escape, achieving further deoxidation and reducing the total oxygen content.

[0032] The soft-blowing time after vacuum breaking (≥15 min) refers to the time after the VD vacuum treatment is completed, the vacuum is broken, and argon gas is introduced at a small flow rate through the permeable bricks at the bottom of the ladle to continue purification. Sufficient soft-blowing time allows fine inclusions still suspended in the molten steel after calcium denaturation treatment and vacuum agitation to slowly float to the top slag and be absorbed under the gentle argon flow. Insufficient time results in incomplete inclusion removal. Excessive argon flow rate will blow open the slag layer, leading to secondary oxidation of the molten steel and gas absorption. Small bubbles generate a stable circulating flow of molten steel, promoting the floating of inclusions without re-entraining already floated inclusions or top slag into the deeper parts of the molten steel. The gentle flow facilitates the collision, aggregation, and growth of tiny inclusions, thus accelerating their floating.

[0033] Preferably, in the continuous casting process, the tundish temperature in the continuous casting section is 1540-1550℃, the protective slag is a CaO-SiO2-TiO2-B2O3 fluorine-free slag system with an alkalinity of 1.1-1.2 and a slag layer thickness of 8-12mm; the crystallizer water flow rate is 105-115Nm³ / h, and the crystallizer electromagnetic stirring current is 350-370A; the secondary cooling water distribution parameters are: specific water flow rate 0.32-0.36L / kg, casting speed 1.4-1.7m / min, and end electromagnetic stirring current 300-400A.

[0034] The tundish is located between the ladle and the crystallizer. The ladle contains a large volume of molten steel, and during the pouring process, its level and pressure constantly change, causing instability in the flow rate of molten steel towards the crystallizer. The tundish maintains a relatively constant liquid level, providing a stable, continuous, and constant-pressure flow of molten steel to the crystallizer.

[0035] The continuous casting section employs a long nozzle in the ladle with argon sealing for protection during casting. The submerged entry nozzle depth is 80-120mm. The long nozzle refers to a tubular device installed below the sliding gate nozzle at the bottom of the ladle when pouring molten steel into the tundish. This allows the molten steel to flow from the ladle through the tubular device into the tundish, completely preventing the molten steel from coming into extensive contact with air during its 1-2 meter drop. Without it, the steel flow would violently agitate the air, causing easily oxidizable elements such as aluminum and titanium in the molten steel to be oxidized into inclusions. Argon sealing refers to the sealing effect achieved by introducing inert argon gas at the connection between the long nozzle and the ladle's sliding gate nozzle mechanism. The argon gas pressure is slightly higher than atmospheric pressure, actively preventing the intake of external air and further preventing oxidation of the molten steel. The submerged entry nozzle depth refers to the depth to which the submerged entry nozzle is inserted below the molten steel surface in the crystallizer when pouring molten steel from the tundish.

[0036] The combination of water volume and casting speed in the crystallizer can ensure that the initial shell of the cast billet grows uniformly and stably.

[0037] Applying electromagnetic stirring in the crystallizer region serves several purposes: the electromagnetic force generated by the stirring breaks down the dendrites at the solidification front, making them the core of equiaxed crystals, thereby expanding the equiaxed crystal zone in the center of the billet and significantly improving central segregation and porosity; it also promotes the flow of molten steel, making the temperature and composition more uniform.

[0038] Specific water consumption refers to the amount of water consumed to cool each kilogram of steel. The purpose of weak cooling is to: reduce the temperature difference between the inside and outside of the billet, reduce thermal stress, and prevent internal cracks; slow down the solidification rate, which is conducive to the floating of bubbles and inclusions in the liquid phase cavity; and avoid excessive phase transformation stress caused by excessively rapid cooling.

[0039] At the end of solidification, when the volume of the liquid phase cavity is about 20-40%, electromagnetic stirring is applied. The core purpose of this stirring is to break up the dendrites in the remaining liquid phase and accelerate the diffusion of the enriched solute liquid phase, thereby greatly reducing or even eliminating macroscopic segregation in the center of the billet; and to promote the flow of unsolidified molten steel and fill the loose voids that have been formed.

[0040] The steel billet has a cross-sectional dimension of 165×165mm.

[0041] Preferably, the heating process includes: preheating at less than 850°C, first-stage heating at 1000-1100°C, second-stage heating at 1100-1180°C, heating at 1160-1200°C for more than 60 minutes, third-stage heating at 1200-1250°C, high-temperature diffusion at 1240-1260°C for more than 70 minutes, and cooling down to 1120-1150°C; the temperature difference of the casting cross-section is ≤20°C.

[0042] This invention improves the heating process for high-alloy, high-strength, and extremely demanding engineering steel with stringent internal quality requirements. The preheating stage involves slow heating, allowing sufficient time for dissolved hydrogen atoms in the billet to diffuse and escape, preventing white spots or hydrogen-induced cracks during subsequent rapid heating or rolling. A large temperature difference exists between the inside and outside of the billet when it enters the furnace. Prolonged slow heating in the low-temperature zone smooths the temperature gradient, avoiding cracking caused by the superposition of thermal stress and residual stress in the billet itself. The first heating stage completes the transformation of pearlite to austenite and initiates preliminary atomic diffusion within the single-phase austenite region, preparing for subsequent high-temperature diffusion. The second heating stage further reduces the temperature difference across the billet cross-section, allowing more alloy carbides to dissolve into the austenite. The third heating stage aims to achieve a highly uniform temperature from the surface to the core of the billet, ensuring effective high-temperature diffusion across the entire cross-section. High-temperature diffusion eliminates dendritic segregation, central segregation, and porosity, and fully dissolves insoluble carbides. The high-temperature diffusion temperature is too high for subsequent rolling processes. By cooling, the austenite grains that grow at high temperatures will stop growing and a suitable starting temperature will be prepared for the subsequent low-temperature high-pressure rolling.

[0043] The rolling process includes: initial rolling temperature 1120-1150℃, pre-finishing rolling temperature 920-980℃, finishing rolling temperature 800-850℃, final rolling temperature 780-800℃, and cooling bed temperature 700-750℃. Roughing is performed using a non-perforated rolling pattern, while final rolling and pre-finishing rolling use an elliptical-round perforated rolling pattern.

[0044] Since time parameters are typically determined naturally by equipment capacity, rolling rhythm, and deformation rate, rather than being actively set, this invention does not limit the time. The purpose of the initial rolling stage is that austenite recrystallizes extremely rapidly at the initial rolling temperature. After each deformation pass, the elongated austenite grains undergo rapid dynamic or static recrystallization, forming new, finer equiaxed grains, and reducing deformation resistance. The pre-finishing rolling stage marks the beginning of rolling in the non-recrystallized zone. The temperature has dropped below the range where microalloyed carbonitrides such as NbC, VC, and VN precipitate in large quantities. These precipitates pin austenite grain boundaries and dislocations, strongly inhibiting recrystallization. During rolling in this temperature range, austenite grains are flattened and elongated but no longer recrystallize, resulting in numerous deformation bands and dislocations within the grains. This stored deformation energy provides a vast number of nucleation sites for ferrite during subsequent phase transformations, which is crucial for obtaining an ultrafine-grained microstructure. The purpose of the finishing rolling stage is to achieve two-phase rolling and deformation-induced phase transformation. At the finishing rolling temperature, austenite begins to transform into ferrite. During two-phase rolling, deformation acts not only on austenite but also on the already precipitated fine ferrite. This greatly refines the ferrite grains and generates a large number of dislocation substructures, significantly improving the material's strength and toughness. The final rolling temperature is precisely controlled at 780-800℃ to ensure that during rapid cooling after rolling, austenite does not precipitate proeutectoid ferrite first, but directly transforms into fine lamellar pearlite or sorbite, which is crucial for obtaining high fatigue strength and good toughness. The upper cooling bed temperature allows the steel to pass through the entire pearlite transformation zone at a slower speed, resulting in uniform pearlite with appropriate lamellar structure.

[0045] In the production process, the time for each stage of the controlled rolling process is related to the mill's capacity, the number of passes, and the temperature drop. According to existing technology, within a defined temperature range, the deformation required to roll the billet to the target finished product thickness is determined. Therefore, those skilled in the art can easily derive the rolling time without specific limitations. For example, rolling a 200mm square billet to an 80mm square billet requires a certain total number of passes and deformation per pass. The reduction per pass is designed based on the mill's maximum allowable rolling force, motor power, and bite angle. Once the number of passes is determined, the travel time of the rolled piece between stands can be obtained by considering the temperature drop range between each stage.

[0046] The temperature of the rolling process determines the degree of phase transformation, thus the present invention can be limited to temperature conditions only.

[0047] The present invention has at least the following beneficial effects:

[0048] This invention eliminates the harmful effects of phosphorus (P) on temperature toughness by abandoning high phosphorus content and adopting Cu-Cr alloys to achieve weather resistance. It shifts from existing alloying compromise strategies to a method combining compositional purification and alloy optimization, no longer relying on the harmful element P. Instead, it achieves steel performance suitable for extreme conditions through purification and superior alloy combinations. While existing technologies rely on high-carbon solid solution strengthening and pearlite strengthening, this invention employs low-carbon combined with Mn solid solution strengthening, V / Nb composite microalloying for fine-grain strengthening, and precipitation strengthening, ensuring higher steel strength while achieving significantly superior low-temperature toughness compared to existing technologies. The engineering steel provided by this invention not only possesses high strength and toughness but is also suitable for both freeze-thaw and rainy environments.

[0049] Generally, Cu-P alloying is used for atmospheric corrosion resistance, while Cr-Ni alloying (for stainless steel) is used for acid and alkali corrosion resistance. Current research indicates the existence of Cu-Cr alloying, with typical compositions including 1.12% (Cu-Cr-Ni); 0.22~0.35% Cu, 0.40~0.60% Cr, 0.13~0.25% Ni; and 0.35% Cu, 0.71% Cr. This invention requires a material with both good weather resistance and low-temperature toughness; therefore, in addition to Cu-Cr, a significant amount of Ni needs to be added. This is why a Cu-Cr-high-Ni alloy design was adopted.

[0050] To achieve higher strength, existing technologies typically require increasing the carbon (C) content. However, increasing C content easily leads to the formation of chromium carbide and vanadium carbide, affecting product toughness and reducing weather resistance. Therefore, this invention employs a low C content design, while simultaneously increasing the manganese (Mn) content and using V and Nb microalloying to achieve the design objective. Microalloying generally uses V, Nb, Ti, and Cr, but Ti microalloying requires increasing the nitrogen (N) content to ensure a Ti / N ratio of approximately 3.4. In the composition design, we require N to be no higher than 60 ppm; therefore, V-Nb is used for microalloying strengthening.

[0051] To meet design requirements, a Cu-Cr-Mn-Ni alloying system was chosen, employing a V-Nb micro-alloying approach. This composite alloying design, characterized by low carbon and high manganese, utilizes Cu-Cr-high Ni alloying while ensuring weather resistance and low-temperature impact performance. To facilitate the implementation of this alloying approach, the process employs an EAF-LF-VD-CC-Furnace-HR (electric arc furnace-ladle refining furnace-vacuum degassing furnace-continuous casting-heating furnace-hot rolling) process to address defects such as segregation caused by high alloy content and surface cracks resulting from Cu-Cr alloying. The alloying process is completed in the LF refining stage, typically in the ladle. The CC stage utilizes "low superheat + weak secondary cooling" to suppress dendrite segregation and surface crack formation, supplemented by electromagnetic stirring and end-of-line light reduction to further reduce the alloy segregation index. In the heating furnace stage, a high-temperature diffusion process is added to further eliminate alloy segregation, improve steel uniformity, and prevent cracking during rolling. Attached Figure Description

[0052] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0053] Figure 1 The low magnification morphology of the continuously cast billet;

[0054] Figure 2 The first metallographic morphology image of the finished product;

[0055] Figure 3 This is the second metallographic structure of the finished product. Detailed Implementation

[0056] To make the objectives, methods, and advantages of the embodiments of the present invention clearer, the methods in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0057] Example 1: An engineering steel for extreme conditions, comprising by weight percentage: C 0.10%, Si 0.17%, Mn 1.60%, Ni 2.5%, Cu 0.35%, Cr 0.35%, V 0.13%, Nb 0.01%, P content 0.005%, S content 0.005%, O content 20ppm, N content 35ppm, Sn content 0.01%, As content 0.01%, and the balance Fe.

[0058] Example 2: An engineering steel for extreme conditions, comprising by weight percentage: C 0.15%, Si 0.37%, Mn 2.00%, Ni 5.0%, Cu 0.55%, Cr 0.55%, V 0.15%, Nb 0.02%, P content 0.008%, S content 0.005%, O content 16ppm, N content 32ppm, Sn content 0.01%, As content 0.01%, and the balance Fe.

[0059] Example 3: An engineering steel for extreme conditions, comprising by weight percentage: C 0.12%, Si 0.20%, Mn 1.80%, Ni 3.75%, Cu 0.45%, Cr 0.45%, V 0.14%, Nb 0.015%, P content 0.005%, S content 0.008%, O content 18ppm, N content 35ppm, Sn content 0.02%, As content 0.008%, and the balance Fe.

[0060] Example 4: A short-process preparation method for the aforementioned engineering steel, characterized by comprising the following steps:

[0061] S100 scrap steel (S%≤0.05%, P%≤0.05%) is graded and classified by size before use, and then cleaned to remove surface oil, dirt, and non-ferrous metal waste and other contaminants. Before entering the furnace, the scrap steel is fed into a horizontal feeding system for preheating. The heat source comes from the collection and utilization of waste gas from electric arc furnace smelting, and the preheating temperature is 800℃. The preheating treatment removes surface oil, paint, galvanized layer and other impurities from the scrap steel, reducing the impact of impurities on the quality of the steel.

[0062] After preheating, the steel is put into an electric arc furnace for melting. After melting, lime and magnesium carbon balls are added to form slag and smelt to obtain primary refined molten steel. The smelting temperature is 1620℃, the smelting time is 30min, and the final carbon content is 0.06wt%. During smelting, a cluster oxygen lance is used to supply oxygen at a rate of 25Nm³ / t steel.

[0063] When the steel reaches 20% vt, 1.5 kg / t of aluminum ferrooxidant is added for pre-deoxidation, based on the minimum internally controlled composition value. Then, ferrosilicon and ferromanganese are added, and finally, 3 kg / t of lime is added, which is completed when the steel reaches 70% vt. The steel tapping process is accompanied by bottom blowing argon and stirring.

[0064] S200, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the primary refined molten steel is heated and refined to obtain secondary refined molten steel; the ladle inlet temperature is 1565℃, the refining time is 40min, the alkalinity is maintained at 3, the white slag is maintained for 18min, and ferrosilicon, low-carbon ferromanganese, ferronickel, copper blocks and ferrochrome alloy are added during refining to finely adjust the composition to the internal control target value.

[0065] In S300, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the secondary refined molten steel is degassed and refined under vacuum, and then fed into a pure calcium wire after cavitation to obtain tertiary refined molten steel. The ladle inlet temperature is 1630℃, the refining time is 20min, the vacuuming period is 4.5min, and the high vacuum holding time is 25min. After cavitation, the calcium wire is fed in at a pure calcium feed rate of 150m / t, the soft blowing time after cavitation is 18min, the argon flow rate is 30NL / min, and the ladle outlet temperature is 1590℃.

[0066] S400 steel, after being refined three times, is processed through continuous casting, heating, and rolling processes to obtain engineering steel.

[0067] In the continuous casting process, the tundish temperature in the continuous casting section is 1540℃, the protective slag is a CaO-SiO2-TiO2-B2O3 fluorine-free slag system with an alkalinity of 1.1 and a slag layer thickness of 8mm; the crystallizer water flow rate is 105Nm³ / h, and the crystallizer electromagnetic stirring current is 350A; the secondary cooling water distribution parameters are: specific water flow rate 0.32L / kg, casting speed 1.4m / min, and end electromagnetic stirring current 300A.

[0068] The heating process includes: preheating at less than 850℃, first stage heating at 1000℃, second stage heating at 1100℃, heating at 1160℃ for more than 60 minutes, third stage heating at 1200℃, high-temperature diffusion at 1240℃ for more than 70 minutes, and cooling down to 1120℃; the temperature difference of the casting cross section is 18.5℃.

[0069] The rolling process includes: initial rolling temperature 1120℃, pre-finishing rolling temperature 920℃, finishing rolling temperature 800℃, final rolling temperature 780℃, and upper cooling bed temperature 700℃. Roughing is performed using a non-perforated rolling pattern, while final rolling and pre-finishing rolling use an elliptical-round perforated rolling pattern.

[0070] The raw material ratio of the CaO-Al2O3-SiO2 ternary low-alkalinity slag system is: 3 kg / t lime, 2.5 kg / t bauxite, 0.2 kg / t aluminum granules, and 0.2 kg / t silicon carbide.

[0071] Example 5: A short-process preparation method for the aforementioned engineering steel, characterized by comprising the following steps:

[0072] S100 scrap steel (S%≤0.05%, P%≤0.05%) is graded and classified by size before use, and then cleaned to remove surface oil, dirt, and non-ferrous metal waste and other contaminants. Before entering the furnace, the scrap steel is fed into a horizontal feeding system for preheating. The heat source comes from the collection and utilization of waste gas from electric arc furnace smelting. The preheating temperature is 1000℃. The preheating treatment removes surface oil, paint, galvanized layer and other impurities from the scrap steel, reducing the impact of impurities on the quality of the steel.

[0073] After preheating, the steel is put into an electric arc furnace for melting. After melting, lime and magnesium carbon balls are added to form slag and smelt to obtain primary refined molten steel. The smelting temperature is 1650℃, the smelting time is 40min, and the final carbon content is 0.08wt%. During smelting, a cluster oxygen lance is used to supply oxygen at a rate of 30Nm³ / t steel.

[0074] When the steel reaches 30% vt, 2.0 kg / t of aluminum ferrooxidant is added for pre-deoxidation, based on the lowest internally controlled composition value. Then, ferrosilicon and ferromanganese are added, and finally 4 kg / t of lime is added, which is completed when the steel reaches 80% vt. The steel tapping process is accompanied by bottom blowing argon and stirring.

[0075] S200, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the primary refined molten steel is heated and refined to obtain secondary refined molten steel; the ladle inlet temperature is 1570℃, the refining time is 50min, the alkalinity is maintained at 4, the white slag is maintained for 18min, and ferrosilicon, low-carbon ferromanganese, ferronickel, copper blocks and ferrochrome alloy are added during refining to finely adjust the composition to the internal control target value.

[0076] In S300, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the secondary refined molten steel is degassed and refined under vacuum, and then fed into a pure calcium wire after cavitation to obtain tertiary refined molten steel. The ladle inlet temperature is 1650℃, the refining time is 30min, the vacuuming period is 4.5min, and the high vacuum holding time is 28min. After cavitation, the calcium wire is fed in at a pure calcium feed rate of 180m / t, the soft blowing time after cavitation is 20min, the argon flow rate is 50NL / min, and the ladle outlet temperature is 1610℃.

[0077] S400 steel, after being refined three times, is processed through continuous casting, heating, and rolling processes to obtain engineering steel.

[0078] In the continuous casting process, the tundish temperature in the continuous casting section is 1550℃, the protective slag is a CaO-SiO2-TiO2-B2O3 fluorine-free slag system with an alkalinity of 1.2 and a slag layer thickness of 12mm; the crystallizer water flow rate is 115Nm³ / h, and the crystallizer electromagnetic stirring current is 370A; the secondary cooling water distribution parameters are: specific water flow rate 0.36L / kg, casting speed 1.7m / min, and end electromagnetic stirring current 400A.

[0079] The heating process includes: preheating at less than 850℃, first stage heating at 1100℃, second stage heating at 1180℃, heating at 1200℃ for more than 60 minutes, third stage heating at 1250℃, high-temperature diffusion at 1260℃ for more than 70 minutes, and cooling down to 1150℃; the temperature difference of the casting cross section is 15℃.

[0080] The rolling process includes: initial rolling temperature 1150℃, pre-finishing rolling temperature 980℃, finishing rolling temperature 850℃, final rolling temperature 800℃, and upper cooling bed temperature 750℃. Roughing is performed using a non-perforated rolling pattern, while final rolling and pre-finishing rolling use an elliptical-round perforated rolling pattern.

[0081] The raw material ratio of the CaO-Al2O3-SiO2 ternary low-alkalinity slag system is: 4 kg / t lime, 3.5 kg / t bauxite, 0.3 kg / t aluminum granules, and 0.3 kg / t silicon carbide.

[0082] Example 6: A short-process preparation method for the aforementioned engineering steel, characterized by comprising the following steps:

[0083] S100 scrap steel (S%≤0.05%, P%≤0.05%) is graded and classified by size before use, and then cleaned to remove surface oil, dirt, and non-ferrous metal waste and other contaminants. Before entering the furnace, the scrap steel is fed into a horizontal feeding system for preheating. The heat source comes from the collection and utilization of waste gas from electric arc furnace smelting. The preheating temperature is 800-1000℃. The preheating treatment removes surface oil, paint, galvanized layer and other impurities from the scrap steel, reducing the impact of impurities on the quality of the steel.

[0084] After preheating, the steel is put into an electric arc furnace for melting. After melting, lime and magnesium carbon balls are added to form slag and smelt to obtain primary refined molten steel. The smelting temperature is 1635℃, the smelting time is 35min, and the final carbon content is 0.07wt%. During smelting, a cluster oxygen lance is used to supply oxygen, and the oxygen supply is 28Nm³ / t steel.

[0085] When the steel reaches 25% vt, 1.8 kg / t of aluminum ferrooxidant is added for pre-deoxidation, based on the minimum internally controlled composition value. Then, ferrosilicon and ferromanganese are added, and finally, 3.5 kg / t of lime is added, which is completed when the steel reaches 75% vt. The steel tapping process is accompanied by bottom blowing argon and stirring.

[0086] S200, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the primary refined molten steel is heated and refined to obtain secondary refined molten steel; the ladle inlet temperature is 1568℃, the refining time is 45min, the alkalinity is maintained at 3.5, the white slag holding time is 17min, and ferrosilicon, low-carbon ferromanganese, ferronickel, copper blocks and ferrochrome alloy are added during refining to finely adjust the composition to the internal control composition target value;

[0087] In S300, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the secondary refined molten steel is degassed and refined under vacuum, and then fed into a pure calcium wire after cavitation to obtain tertiary refined molten steel. The ladle inlet temperature is 1640℃, the refining time is 25min, the vacuuming period is 4.5min, and the high vacuum holding time is 23min. After cavitation, the calcium wire is fed in at a pure calcium feed rate of 165m³ / t, the soft blowing time after cavitation is 22min, the argon flow rate is 40NL / min, and the ladle outlet temperature is 1600℃.

[0088] S400 steel, after being refined three times, is processed through continuous casting, heating, and rolling processes to obtain engineering steel.

[0089] In the continuous casting process, the tundish temperature in the continuous casting section is 1545℃, the protective slag is a CaO-SiO2-TiO2-B2O3 fluorine-free slag system with an alkalinity of 1.2 and a slag layer thickness of 10mm; the crystallizer water flow rate is 110Nm³ / h, and the crystallizer electromagnetic stirring current is 360A; the secondary cooling water distribution parameters are: specific water flow rate 0.34L / kg, casting speed 1.55m / min, and end electromagnetic stirring current 350A.

[0090] The heating process includes: preheating at less than 850℃, first stage heating at 1050℃, second stage heating at 1140℃, heating at 1180℃ for more than 60 minutes, third stage heating at 1225℃, high-temperature diffusion at 1250℃ for more than 70 minutes, and cooling down to 1135℃; the temperature difference of the casting cross section is 15℃.

[0091] The rolling process includes: initial rolling temperature 1135℃, pre-finishing rolling temperature 950℃, finishing rolling temperature 825℃, final rolling temperature 790℃, and upper cooling bed temperature 725℃. Roughing is performed using a non-perforated rolling pattern, while final rolling and pre-finishing rolling use an elliptical-round perforated rolling pattern.

[0092] The raw material ratio of the CaO-Al2O3-SiO2 ternary low-alkalinity slag system is: 3.5 kg / t lime, 3 kg / t bauxite, 0.25 kg / t aluminum granules, and 0.25 kg / t silicon carbide.

[0093] Experiment: After preparing engineering steel according to the components provided in Examples 1-3 and the preparation methods provided in Examples 4-6, performance tests were conducted, and the test results are shown in Table 1.

[0094] Test Example 1.4 refers to the engineering steel prepared using the composition provided in Example 1 and the preparation method provided in Example 4, and so on.

[0095] The difference between Comparative Example 1 and Experimental Example 3.6 is that the component does not contain V.

[0096] The difference between Comparative Example 2 and Experimental Example 3.6 is that the component does not contain Nb.

[0097] The difference between Comparative Example 3 and Experimental Example 3.6 is that Cr accounts for 0.2% of the component.

[0098] The difference between Comparative Example 4 and Experimental Example 3.6 is that C accounts for 0.2% of the component.

[0099] The difference between Comparative Example 5 and Experimental Example 3.6 is that Mn accounts for 1.4% of the component.

[0100] Table 1

[0101] Experimental Example 1.4 Experimental Example 1.5 Experimental Example 1.6 Experimental Example 2.4 Experimental Example 2.5 tensile strength 735 810 770 720 795 Yield strength 634 710 665 615 690 elongation 27.5 25.0 24.0 28.5 24.0 Impact power 35 28 32 37 29 Salt spray test No red rust No red rust No red rust No red rust No red rust Experimental Example 2.6 Experimental Example 3.4 Experimental Example 3.5 Experimental Example 3.6 Comparative Example 1 tensile strength 755 740 825 780 745 Yield strength 650 645 725 678 634 elongation 26.0 26.5 21.5 24.5 26.0 Impact power 33 31 28 30 38 Salt spray test No red rust No red rust No red rust No red rust No red rust Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 tensile strength 760 762 880 750 Yield strength 660 662 770 645 elongation 23.5 25.0 18.0 26.5 Impact power 24 33 14 36 Salt spray test No red rust A small amount of red rust No red rust No red rust

[0102] The unit of tensile strength Rm is MPa, and the unit of yield strength R p0.2 The unit is MPa, and the unit of impact energy is J (sample size: 55×10×10mm, notch type: Charpy V notch, test temperature: -60℃). The neutral salt spray test (NSS) lasts for 48 hours.

[0103] As can be seen from the test results of the experimental examples, the engineering steel provided by the present invention has good mechanical strength, low temperature resistance and weather resistance.

[0104] The low-magnification morphology of the continuously cast billet corresponding to Experiment Example 3.6 is shown below. Figure 1 Low-magnification inspection results of continuously cast billets: central equiaxed crystal ratio: can be increased to over 35%; central carbon segregation index: can be stably controlled below 1.05; central shrinkage / porosity is significantly improved, and density is increased.

[0105] The metallographic morphology of the finished product corresponding to Experimental Example 3.6 is shown below. Figure 2-3 Finished product inspection results: The metallographic structure consists of fine-grained ferrite + pearlite + a small amount of bainite, with a ferrite grain size of 8.0~9.5.

[0106] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An engineered steel for use in extreme conditions, characterized in that, By weight percentage, it includes: C 0.10%-0.15%, Si 0.17%-0.37%, Mn 1.60%-2.00%, Ni 2.5%-5.0%, Cu 0.35%-0.55%, Cr 0.35%-0.55%, V 0.13%-0.15%, Nb 0.01-0.02%.

2. The engineering steel according to claim 1, characterized in that, By weight percentage, P content ≤ 0.015%, S content ≤ 0.015%, O content ≤ 30 ppm, and N content ≤ 60 ppm.

3. The engineering steel according to claim 1 or 2, characterized in that, Ferrite grain size 8.0-9.5; yield strength ≥600MPa; tensile strength ≥700MPa; impact energy at -60℃ ≥27J.

4. A short-process preparation method for engineering steel according to any one of claims 1-3, characterized in that, Includes the following steps: After preheating S100 scrap steel, it is fed into an electric arc furnace, melted, and then lime and magnesium carbon balls are added to form slag for smelting to obtain primary refined molten steel. S200, under the CaO-Al2O3-SiO2 ternary low-basicity slag system, primary refined molten steel is heated and refined to obtain secondary refined molten steel; S300, under the CaO-Al2O3-SiO2 ternary low-alkalinity slag system, the secondary refined molten steel is degassed and refined under vacuum, and then fed into a pure calcium wire to obtain the tertiary refined molten steel. S400 steel, after being refined three times, is processed through continuous casting, heating, and rolling processes to obtain engineering steel. The weight parts of the components in the CaO-Al2O3-SiO2 ternary low-alkalinity slag system are: CaO 55-60 parts, SiO2 15-20 parts, Al2O3 10-15 parts, TiO2 5-10 parts, and MgO 8-12 parts.

5. The short-process preparation method according to claim 4, characterized in that, In step S100, the preheating temperature is 800-1000℃, the smelting temperature is 1620-1650℃, the smelting time is 30-40min, the final carbon content is 0.06wt%-0.08wt%, and the oxygen supply during smelting is 25-30Nm³ / t steel.

6. The short-process preparation method according to claim 4, characterized in that, In step S100, when refining and tapping the steel, when the steel reaches 20wt%-30wt%, 1.5-2.0 kg / t of ferroaluminum, ferrosilicon, ferromanganese and 3-4 kg / t of lime are added sequentially, based on the lowest internally controlled composition value. The addition is completed when the steel reaches 70wt%-80wt%. The tapping process is accompanied by bottom blowing argon gas and stirring.

7. The short-process preparation method according to claim 4, characterized in that, In step S200, the ladle inlet temperature is ≥1560℃, the refining temperature is 1630-1650℃, the refining time is 40-50min, the basicity is maintained at 3-4, the white slag retention time is ≥15min, and ferrosilicon, low-carbon ferromanganese, ferronickel, copper blocks and ferrochrome alloy are added during refining to finely adjust the composition to the internal control composition target value.

8. The short-process preparation method according to claim 5, characterized in that, In step S300, the ladle inlet temperature is 1630-1650℃, the refining time is 20-30 min, the vacuuming period is ≤6 min, and the high vacuum holding time is ≥20 min; after venting, calcium wire is fed, the pure calcium feed rate is 150-180 m / t, the soft blowing time after venting is ≥15 min, the argon flow rate is 30-50 NL / min, and the ladle outlet temperature is 1590-1610℃.

9. The short-process preparation method according to any one of claims 4-8, characterized in that, In the continuous casting process, the tundish temperature in the continuous casting section is 1540-1550℃. The protective slag is a CaO-SiO2-TiO2-B2O3 fluorine-free slag system with the following weight proportions: CaO 35-40 parts, SiO2 25-30 parts, B2O3 5-10 parts, and TiO2 5-15 parts. The basicity is 1.1-1.2, and the slag layer thickness is 8-12mm. The crystallizer water flow rate is 105-115 Nm³ / h, and the crystallizer electromagnetic stirring current is 350-370A. The secondary cooling water distribution parameters are: specific water flow rate 0.32-0.36 L / kg, casting speed 1.4-1.7 m / min, and end electromagnetic stirring current 300-400A.

10. The short-process preparation method according to claim 9, characterized in that, The heating process includes: preheating at less than 850℃ for 25-35 minutes, first-stage heating at 1000-1100℃ for 35-45 minutes, second-stage heating at 1100-1180℃ for 45-60 minutes, third-stage heating at 1160-1200℃ for 35-45 minutes, third-stage heating at 1200-1250℃ for 35-45 minutes, high-temperature diffusion at 1240-1260℃ for more than 70 minutes, and cooling down to 1120-1150℃; the temperature difference of the casting cross-section is ≤20℃.

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