Preparation method of 700mpa girder steel with excellent low temperature toughness for alpine region

Abstract

The application discloses a preparation method of 700MPa girder steel with excellent low-temperature toughness for high-cold regions, and belongs to the technical field of steel metallurgy, and comprises the following steps: S1, selecting a low-carbon mode for converter smelting, suppressing nitrogen increase through top and bottom combined blowing to strengthen the molten pool stirring, performing slag washing when tapping, and forming slag in advance; S2, LF refining, controlling nitrogen under 0.2MPa micro-positive pressure, creating low-melting-point refining slag and extreme desulfurization, removing large-size non-metallic inclusions, and performing modification treatment on residual MnS and Al2O3 by calcium treatment to remove small-size non-metallic inclusions; S3, full-process protection pouring of slab continuous casting, optimizing the electromagnetic stirring of the crystallizer, dynamic soft reduction, and cooling capacity of the secondary cooling zone under the premise of low superheat, and shortening TiN precipitation time; and S4, sequentially performing heating, rough rolling and finish rolling, and performing finish rolling in the austenite region, and rapidly cooling high-temperature austenite by low coiling, so that TiN is precipitated in a finer and more dispersed form.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and in particular relates to a method for preparing 700MPa beam steel with excellent low-temperature toughness for use in high-altitude and cold regions. Background Technology

[0002] 700MPa high-strength beam steel is mainly used for the longitudinal beams, cross beams and chassis structures of heavy-duty trucks and commercial vehicles, as well as irregular structural components such as pump truck booms and crane booms. It should have excellent plasticity and low-temperature toughness to ensure structural safety in high-altitude and cold regions.

[0003] Technical bottlenecks in the application of 700MPa high-strength beam steel in cold regions: (1) Bending corner cracks: Due to the presence of large-sized TiN non-metallic inclusions in the original slab, the cracks expand under the stamping tensile stress; (2) Stamping cracks: Due to the coarsening of the matrix grains, stress concentration occurs at the wire hole of the longitudinal beam during the continuous stamping process, causing cracks; (3) Low-temperature toughness deterioration in the heat-affected zone: Grain coarsening in the welding heat-affected zone leads to structural fracture.

[0004] In summary, how to eliminate TiN non-metallic inclusions, matrix grain coarsening, and low-temperature toughness deterioration in the heat-affected zone are the technical bottlenecks for the precipitous decline in low-temperature impact toughness of 700MPa high-strength beam steel in cold regions.

[0005] The reason for the precipitous decrease in low-temperature impact toughness of 700MPa high-strength beam steel in cold regions:

[0006] (1) Large-size TiN nonmetallic inclusions: First, nitrogen is controlled throughout the steelmaking process to reduce TiN formation; second, in the later stage of solidification, the size of TiN is controlled by the cooling rate and the segregation of nitrogen and titanium elements, that is, the time for supersaturated titanium and nitrogen elements in the molten steel to precipitate TiN during cooling is shortened in the high-temperature stage; finally, under the slow cooling conditions of the rolling process, Ti and N atoms have sufficient time to diffuse and aggregate to form coarse precipitates >10μm, while rapid cooling restricts atomic migration, causing TiN to precipitate in a finer and more dispersed form. The average equivalent diameter of TiN decreases from 10.93μm to about 0.77μm. The difference in thermal expansion coefficient between large-size TiN nonmetallic inclusions and the matrix leads to the generation of microcracks at the interface, which exacerbates the risk of crack initiation at low temperatures and causes early fatigue cracking in key parts such as the web of the beam.

[0007] (2) Matrix grain coarsening: On the one hand, higher superheated casting increases the temperature difference between the liquid steel and the solidified shell, inhibiting the preferential growth of equiaxed crystals, reducing the cooling rate of the secondary cooling zone, decreasing the temperature gradient, and leading to a columnar crystal-dominated structure due to rapid solidification. Electromagnetic stirring does not effectively increase the cutting speed of the billet cross-section, inhibiting the growth of equiaxed crystals and expanding the columnar crystal zone. On the other hand, slow cooling during rolling reduces the supercooling of crystallization, causing the nucleation rate to increase at a rate lower than the grain growth rate, thus resulting in matrix grain coarsening. As a result, the matrix grain coarsening causes stress concentration at the threading hole of the longitudinal beam during the continuous stamping process, leading to cracking.

[0008] (3) Deterioration of low-temperature toughness in the heat-affected zone: The welding process generates high-temperature thermal cycling, Nb and Ti elements are redissolved, and precipitated and coarsened at the grain boundaries during cooling. The surge in the number of precipitated phases significantly reduces the low-temperature impact toughness of the heat-affected zone, resulting in a 50% to 60% loss of impact energy at -40℃ in the heat-affected zone compared to the impact energy at -40℃ in the matrix, which seriously deteriorates the low-temperature impact toughness of the heat-affected zone.

[0009] In summary, to break through the high cost barrier of 700MPa beam steel, a low-cost, high-strength beam steel with excellent low-temperature toughness suitable for cold regions is disclosed. This method makes a significant technological breakthrough in eliminating three technical bottlenecks: TiN non-metallic inclusions, matrix grain coarsening, and low-temperature toughness deterioration in the heat-affected zone. Specifically, it achieves nanoscale TiN or Ti(C,N) precipitates with diffuse distribution, extremely fine and uniform grain size, and extremely high low-temperature toughness in the matrix.

[0010] The patent application filed by Zhang Zhibo et al., entitled "A 700MPa Grade Hot-Rolled High-Strength Beam Steel with Good Low-Temperature Toughness and Its Manufacturing Method," adds 0.10% to 0.15% Ti. However, it does not mention how to control TiN precipitation during steelmaking or how to control TiN precipitation during rolling cooling, thus failing to address the negative impact on low-temperature toughness. Instead, it intentionally adds precious metals Ni (0.01% to 1.0%) and Nb (0.025% to 0.065%) to improve grain refinement and enhance low-temperature impact toughness. Examples mention a low-temperature impact toughness of 90J and 110J at -40℃. However, the cost is too high, which is not conducive to large-scale production by most enterprises in a market economy.

[0011] The application submitted by Yang Weiyu et al., titled "A Method for Improving the Low-Temperature Impact Toughness of 700MPa Grade Automotive Beam Steel with a Thickness of 10-18mm," states that Mn is 1.70%-1.90%, indicating excessive manganese segregation in the billet, but fails to mention the segregation rating at the billet center. Nb is 0.025%-0.035%, Ti is 0.07%-0.09%, and N ≤ 0.0020%. The Nb precipitation strengthening method replaces some of the titanium precipitation strengthening, reducing the probability of TiN non-metallic inclusion precipitation, which is beneficial for improving low-temperature impact toughness. The half-thickness grain size is grade 12-15, while GB / T6394-2017 specifies grade 14 as the highest grade; the claim of grade 15 contradicts well-known scientific principles. In summary, adding Nb and RH refining to remove nitrogen increases steelmaking costs and is detrimental to the large-scale production of most steel companies in a market economy.

[0012] The patents filed by Wang Changsheng et al., "A Thin-gauge 700BL Beam Steel Without Nb and Its Production Method," and by Zhang Zhigang et al., "A Production Method of an Economical and Easily Formable Rare Earth Ti Microalloyed 700MPa Grade Beam Steel Strip," do not mention low-temperature impact toughness, nor do they mention how to control TiN precipitation in the steelmaking process or how to control TiN precipitation during rolling and cooling, and the negative impact on low-temperature toughness. Based on the composition and controlled rolling and cooling capabilities in the invention descriptions, the mechanical properties meet the process requirements, but the actual low-temperature toughness control capability is questionable, and neither patent is suitable for service environments in high-altitude and cold regions.

[0013] The application by Yang Weiyu et al., entitled "A Thick-Spec High-Toughness 700MPa Grade Automotive Beam Steel and Its Preparation Method", intentionally adds precious metals Mo: 0.06%~0.12% and Nb: 0.025%~0.075%, which enhances low-temperature impact toughness and improves fine-grain strengthening. The example mentions a low-temperature impact toughness of 80~140J at -40℃, but the cost is too high, which is not conducive to the large-scale production of most enterprises under the market economy.

[0014] The application filed by Li Xiangqian et al., entitled "A Low-Temperature High-Toughness 700MPa High-Strength Beam Steel and Its Production Method", intentionally adds precious metals Ni: 0.12%~0.20%, Cr: 0.15%~0.30% and Nb: 0.035%~0.055% to enhance low-temperature impact toughness and improve grain refinement. The examples mention a low-temperature impact toughness of 86.5J and 75.5J at -40℃. However, the cost of precious metals is too high, which is not conducive to the large-scale production of most enterprises under the market economy.

[0015] The application by Zhou Yanfeng et al., entitled "A 1.2-2.0mm thin-gauge 700MPa grade hot-rolled beam steel and its preparation method", intentionally adds precious metal rare earth Re≤0.01%, which can refine the grain and improve the low-temperature impact toughness. However, it does not mention the actual low-temperature toughness control level, and whether it is suitable for service environments in high-altitude and cold regions is questionable. In addition, the cost of rare earth is too high, which is not conducive to the large-scale production of most enterprises under the market economy.

[0016] The application submitted by Zhang Zhigang et al., entitled "A Low-Cost Method for Preparing Thin-Grade 700MPa Grade Steel Strip for Automobile Beams with Ti Microalloying," reduces costs by 95 yuan / ton, resulting in significant cost reduction. The mechanical properties meet the process requirements. However, it adds RH cycle degassing treatment, which increases production costs accordingly. Furthermore, it does not mention the control level of low-temperature impact toughness, and whether it is suitable for service environments in high-altitude and cold regions remains to be discussed. Summary of the Invention

[0017] This invention addresses the technical bottleneck of the precipitous decline in low-temperature impact toughness of 700MPa high-strength beam steel in cold regions, and proposes a method for preparing 700MPa beam steel with excellent low-temperature toughness for cold regions.

[0018] The preparation method of 700MPa beam steel with excellent low-temperature toughness for high-altitude and cold regions includes: S1. Select the low-carbon mode for converter smelting, strengthen the stirring of the molten pool by top and bottom blowing to suppress nitrogen increase, wash the slag after tapping, and form slag in advance. S2 and LF refining, nitrogen control under 0.2MPa micro-positive pressure, to create low melting point refining slag and extreme desulfurization, to remove large non-metallic inclusions, calcium treatment to modify residual MnS and Al2O3, to remove small non-metallic inclusions. S3. Slab continuous casting with full protection pouring, under the premise of low superheat, optimize the crystallizer electromagnetic stirring, dynamic light pressure, and cooling capacity of the secondary cooling zone to shorten the TiN precipitation time. S4. The heating, roughing and finishing processes are carried out in sequence. The finishing process is carried out in the austenite region. The low coiling rapidly cools the high-temperature austenite, which promotes the precipitation of TiN in a finer and more dispersed form.

[0019] Preferably, S1 includes: The converter was turned over, resulting in a carbon content of 0.03% to 0.04%. By strengthening the stirring of the molten pool through top and bottom blowing, the carbon-oxygen product is no more than 0.0025%, and the dissolved oxygen content in the steel is 0.0625% to 0.0833%. CO bubbles and bottom-blown Ar bubbles form a local low-pressure zone, which reduces the nitrogen partial pressure and accelerates the diffusion and removal of nitrogen atoms into the bubbles. During the slag washing process, 0.1 kg / t to 0.2 kg / t of carbon raiser for pre-deoxidation, 2.5 kg / t to 3.0 kg / t of lime, 1.5 kg / t to 2.0 kg / t of aluminum blocks, and 0.15 kg / t to 0.25 kg / t of calcium carbide are added sequentially for deep deoxidation to ensure that the oxygen content is no more than 0.0020%.

[0020] Preferably, S2 includes: Molten steel is refined in the LF furnace, with the dust removal opening maintained at 5% to 10%. The LF refining process is carried out under a slightly positive pressure of 0.2 MPa. Calcium carbide and silicon carbide undergo composite deoxidation to generate CO, which forms a CO protective layer on the water-cooled furnace cover to reduce nitrogen accumulation. The total amount of lime in the converter and LF furnace is 6.0 kg / t to 7.0 kg / t, the amount of calcium carbide in the converter and LF furnace is 0.4 kg / t to 0.6 kg / t, the amount of silicon carbide in the LF furnace is 0.1 kg / t to 0.2 kg / t, and the amount of ferrosilicon powder in the LF furnace is 0.5 kg / t for composite diffusion deoxidation and extreme desulfurization. The amount of sulfur in the LF furnace is no more than 0.003%, which produces low-melting-point refining slag to adsorb large-sized non-metallic inclusions. The pure calcium line modifies the residual MnS and Al2O3 at a speed of 3.0 m / s to 4.0 m / s, and small-sized non-metallic inclusions are removed by the low-melting-point refining slag.

[0021] Preferably, S3 includes: Open the argon pipeline at the long nozzle, with an argon flow rate of 30NL / min to 100NL / min. The argon flow rate at the tundish nozzle is 2NL / min to 5NL / min. The argon flow rate for the stopper rod is 2NL / min to 5NL / min. The argon flow rate for the interplate is 2NL / min to 5NL / min. By improving the nozzle insertion depth to 150mm-160mm, optimizing the crystallizer electromagnetic stirring at 350A / 7Hz, dynamic light pressing down to 4.0mm-4.2mm, and the secondary cooling zone water volume to 0.6L / kg-0.8L / kg under the premise of superheated temperature of 10℃-25℃, the precipitation time of supersaturated titanium and nitrogen elements in the molten steel during cooling is shortened, and the slab center segregation reaches C1.0.

[0022] Preferably, S4 includes: The finishing rolling temperature is 850℃~880℃. It utilizes the high plastic deformation capacity of austenite, while refining austenite grains through dynamic recrystallization and suppressing austenite grain growth through rapid cooling in the laminar flow cooling front section. By rapidly cooling the high-temperature austenite to the coiling temperature range of 560℃ to 600℃ at a cooling rate of 20℃ / s to 30℃ / s, the pearlite transformation is suppressed. Under rapid cooling conditions, Ti and N atoms do not have sufficient time to diffuse and aggregate, thus limiting atomic migration and promoting the precipitation of TiN in a finer and more dispersed form.

[0023] Preferred options also include: S5. Through sampling and testing, the mechanical properties, grain size, and low-temperature toughness of the 700MPa high-strength beam steel were determined.

[0024] Preferably, the mechanical properties include: yield strength, tensile strength and elongation.

[0025] Preferably, the yield strength of the high-strength beam steel with low-temperature toughness of 700MPa is not less than 650MPa, the tensile strength is not less than 760MPa, and the elongation is not less than 20%.

[0026] Preferably, the grain size of the high-strength beam steel with a low-temperature toughness of 700MPa is not less than grade 12.5; the impact toughness at -20℃ is not less than 150J, and the impact toughness at -40℃ is not less than 80J.

[0027] Preferably, the chemical composition of the high-strength beam steel with low-temperature toughness of 700MPa, by mass percentage, includes C: 0.04%~0.08%, Si≤0.20%, Mn: 1.40%~1.60%, P≤0.020%, S≤0.005%, Al0.02%~0.06%, Ti: 0.100%~0.150%, O≤0.002%, N≤0.0040%, with the balance being Fe and unavoidable impurities.

[0028] The present invention has the following beneficial effects: This invention addresses the technical bottleneck of the precipitous decline in low-temperature impact toughness of 700MPa high-strength beam steel in cold regions. It employs a comprehensive nitrogen control system from converter to LF: fully utilizing the converter's low-blowing denitrification capability, and maintaining LF refining under a slight positive pressure of 0.2MPa, forming a gas protective layer in the water-cooled furnace cover to reduce nitrogen accumulation. The slag system from converter to LF is optimized: the slag washing structure and addition sequence are optimized for earlier slag formation and deep deoxidation; the LF refining slag-forming material and deoxidizer structure are optimized for rapid low-melting-point refining slag, achieving extreme desulfurization and removal of non-metallic inclusions; and the slab continuous casting process is protected throughout, with comprehensive process optimization including improved nozzle insertion depth, optimized crystallizer electromagnetic stirring, dynamic light pressure, and secondary cooling capacity under low superheat conditions. This shortens the time for supersaturated titanium and nitrogen elements in the molten steel to precipitate TiN during cooling, achieving C1.0 segregation at the slab center. After heating, rough rolling, and finishing rolling at a temperature of 850℃~880℃, the rolling process is located in the austenite region. This allows for full utilization of the high plasticity of austenite. At the same time, dynamic recrystallization refines the austenite grains. Rapid cooling in the laminar flow cooling stage inhibits austenite grain growth. The high-temperature austenite is rapidly cooled to the coiling temperature range of 560℃~600℃ at a cooling rate of 20-30℃ / s, which suppresses pearlite transformation. Under rapid cooling conditions, Ti and N atoms do not have sufficient time to diffuse and aggregate, thus limiting atomic migration and promoting the precipitation of TiN in a finer and more dispersed morphology. This patented high-strength beam steel with excellent low-temperature toughness (700MPa) and yield strength ≥650MPa, tensile strength ≥760MPa, elongation ≥20%, and mechanical properties superior to national standards; grain size ≥12.5 grade; and low-temperature impact toughness ≥150J at -20℃ and ≥80J at -40℃, is produced at low cost. Compared with Nb and Ti microalloyed designs, this patented design reduces costs by ≥200 yuan / ton. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a comparative example of the core tissue structure of a preferred embodiment of the present invention; Figure 2 This is a diagram of the core structure of a preferred embodiment of the present invention; Figure 3 This is a flowchart of a preferred embodiment of the present invention. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Please see Figures 1 to 3 As shown, the preparation method of 700MPa beam steel with excellent low-temperature toughness for high-altitude and cold regions includes the following.

[0033] S1. Select the low-carbon mode for converter smelting, strengthen the stirring of the molten pool by top and bottom blowing to suppress nitrogen increase, wash the slag after tapping, and form slag in advance. In the converter smelting process, a low-carbon operating mode is selected to optimize smelting efficiency and control carbon content, thereby reducing environmental pollution and improving steel quality. By employing top-and-bottom blowing technology, gas is simultaneously blown from the top and bottom of the converter, enhancing the stirring effect of the molten pool, promoting reaction uniformity, and effectively suppressing nitrogen dissolution and nitrogen accumulation, thus reducing the nitrogen content in the steel, preventing material embrittlement, and improving the mechanical properties of the final product. Furthermore, slag washing is performed during the tapping stage, utilizing the chemical properties of the slag for cleaning and refining, forming active slag in advance, which facilitates desulfurization and dephosphorization reactions, accelerates the slag formation process, improves smelting efficiency, and ensures the purity and stability of the molten steel.

[0034] S2 and LF refining, nitrogen control under 0.2MPa micro-positive pressure, to create low melting point refining slag and extreme desulfurization, to remove large non-metallic inclusions, calcium treatment to modify residual MnS and Al2O3, to remove small non-metallic inclusions. During the LF refining process, nitrogen content is precisely controlled under a slight positive pressure of 0.2 MPa to prevent nitrogen accumulation in the molten steel while promoting the removal of harmful gases. By formulating low-melting-point refining slag, slag fluidity is optimized, enhancing the desulfurization reaction kinetics and achieving optimal desulfurization. Strong stirring is employed to promote the flotation and separation of large-sized non-metallic inclusions, purifying the molten steel. Subsequently, calcium treatment is performed to modify the morphology of residual MnS inclusions and Al2O3 clusters, transforming them into low-melting-point calcium aluminates or spherical sulfur oxides, improving the rollability and toughness of the steel. Finally, processes such as soft argon blowing are used to further remove small-sized non-metallic inclusions, comprehensively improving the cleanliness of the molten steel.

[0035] S3. The entire process of slab continuous casting is protected during pouring. Under the premise of low superheat, the electromagnetic stirring, dynamic light pressure and cooling capacity of the crystallizer are optimized to shorten the TiN precipitation time. Specifically, the TiN precipitation time is shortened from 800 seconds to 630 seconds, effectively suppressing TiN precipitation. During the continuous casting of slabs, full-process protective casting is implemented to effectively prevent steel oxidation and slag entrapment. At the same time, under low superheat process conditions, the electromagnetic stirring parameters of the crystallizer are optimized to enhance the fluidity and homogenization of the molten steel. Dynamic light reduction technology is applied to precisely control the solidification process of the slab and reduce internal cracks. The cooling capacity of the secondary cooling zone is improved to achieve efficient heat exchange and solidification control, thereby significantly shortening the precipitation time of titanium nitride (TiN) inclusions and improving the internal quality of the slab and the performance of the final product.

[0036] S4. The heating, roughing and finishing processes are carried out in sequence. The finishing process is carried out in the austenite region. The low coiling rapidly cools the high-temperature austenite, which promotes the precipitation of TiN in a finer and more dispersed form.

[0037] First, the slab is heated to a uniform austenitizing temperature, providing a good microstructure for subsequent rolling. The heated slab then undergoes two stages: rough rolling and finish rolling. Rough rolling is primarily used to significantly reduce the slab thickness and achieve initial shaping, while finish rolling is performed within the austenite recrystallization temperature range, a process belonging to the austenitic rolling region. Through multiple passes of continuous deformation, it effectively refines the austenite grains. Subsequently, a lower coiling temperature is used, rapidly cooling the high-temperature austenite below the coiling temperature. This significantly inhibits grain growth and increases the phase transformation driving force, thereby promoting the precipitation of TiN (titanium nitride) particles in a finer, more dispersed form, significantly improving the material's strength, toughness, and resistance to aging.

[0038] To better understand the technical solution of the present invention, the following non-limiting description is provided: S1 includes: The converter was turned over, resulting in a carbon content of 0.03% to 0.04%. By strengthening the stirring of the molten pool through top and bottom blowing, the carbon-oxygen product is no more than 0.0025%, and the dissolved oxygen content in the steel is 0.0625% to 0.0833%. This effectively prevents external nitrogen from penetrating into the molten steel. The CO bubbles and bottom-blown Ar bubbles form a local low-pressure zone, which significantly reduces the nitrogen partial pressure and accelerates the diffusion and removal of nitrogen atoms into the bubbles. During the slag washing process, 0.1 kg / t to 0.2 kg / t of carburizing agent for pre-deoxidation, 2.5 kg / t to 3.0 kg / t of lime, 1.5 kg / t to 2.0 kg / t of aluminum blocks, and 0.15 kg / t to 0.25 kg / t of calcium carbide are added sequentially. Deep deoxidation ensures that the oxygen content is no more than 0.0020%, and early slag formation provides a kinetic and thermodynamic basis for LF.

[0039] S2 includes: Molten steel is refined in the LF furnace, with the dust removal opening maintained at 5% to 10%. The LF refining process is carried out under a slightly positive pressure of 0.2 MPa. Calcium carbide and silicon carbide undergo composite deoxidation to generate CO, which forms a CO protective layer on the water-cooled furnace cover to reduce nitrogen accumulation. The total amount of lime in the converter and LF furnace is 6.0 kg / t to 7.0 kg / t, the amount of calcium carbide in the converter and LF furnace is 0.4 kg / t to 0.6 kg / t, the amount of silicon carbide in the LF furnace is 0.1 kg / t to 0.2 kg / t, and the amount of ferrosilicon powder in the LF furnace is 0.5 kg / t for composite diffusion deoxidation and extreme desulfurization. The amount of sulfur in the LF furnace is no more than 0.003%, which produces low-melting-point refining slag to adsorb large-sized non-metallic inclusions. The pure calcium line modifies the residual MnS and Al2O3 at a speed of 3.0 m / s to 4.0 m / s, and small-sized non-metallic inclusions are removed by the low-melting-point refining slag.

[0040] S3 includes: The slab is continuously cast under full protection. The argon pipeline at the long nozzle is opened, and the argon flow rate is 30NL / min to 100NL / min. The argon flow rate at the tundish nozzle is 2NL / min to 5NL / min. The argon blowing flow rate at the stopper rod is 2NL / min to 5NL / min. The argon blowing flow rate between plates is 2NL / min to 5NL / min. By improving the nozzle insertion depth to 150mm-160mm, optimizing the crystallizer electromagnetic stirring at 350A / 7Hz under the premise of superheat of 10℃-25℃, the dynamic light pressure reduction of 4.0mm-4.2mm, and the water volume in the secondary cooling zone of 0.6L / kg-0.8L / kg, the process optimization shortens the time for supersaturated titanium and nitrogen elements in the molten steel to precipitate TiN during cooling in the high-temperature stage, and the slab center segregation reaches C1.0.

[0041] S4 includes: Heating, roughing and finishing are performed sequentially. The finishing rolling temperature is 850℃~880℃, which is rolled in the austenite region. This can make full use of the high plastic deformation capacity of austenite. At the same time, the austenite grains are refined by dynamic recrystallization, and the austenite grain growth is suppressed by rapid cooling in the laminar flow cooling front section. By rapidly cooling the high-temperature austenite to the coiling temperature range of 560℃ to 600℃ at a cooling rate of 20℃ / s to 30℃ / s, the pearlite transformation is suppressed. Under rapid cooling conditions, Ti and N atoms do not have sufficient time to diffuse and aggregate, thus limiting atomic migration and promoting the precipitation of TiN in a finer and more dispersed form.

[0042] It also includes S5: after sampling and testing, the yield strength is ≥650MPa, the tensile strength is ≥760MPa, the elongation is ≥20%, and the mechanical properties are better than the national standard; the grain size is ≥12.5 grade; the low-cost, high-strength beam steel with excellent low-temperature toughness of 700MPa has a low-temperature impact toughness of ≥150J at -20℃ and ≥80J at -40℃; this patent reduces the cost by ≥200 yuan / ton compared with the Nb and Ti microalloy design.

[0043] The chemical composition of the high-strength beam steel with low-temperature toughness of 700MPa, by mass percentage, includes C: 0.04%~0.08%, Si≤0.20%, Mn: 1.40%~1.60%, P≤0.020%, S≤0.005%, Al: 0.02%~0.06%, Ti: 0.100%~0.150%, O≤0.002%, N≤0.0040%, with the balance being Fe and unavoidable impurities.

[0044] This invention is suitable for the entire process of steelmaking, continuous casting, and rolling. Nitrogen control throughout the converter to LF process: Fully utilizes the converter's low-blowing denitrification capability, with LF refining under a slightly positive pressure of 0.2 MPa, forming a gas protective layer on the water-cooled furnace cover to reduce nitrogen accumulation. Slag system optimization from converter to LF: Optimizes the slag washing structure and addition sequence for earlier slag formation and deep deoxidation; optimizes the slag-forming material and deoxidizer structure in LF refining, resulting in rapid, low-melting-point refining slag for extreme desulfurization and removal of non-metallic inclusions; Protective casting throughout the slab continuous casting process, through improved nozzle insertion depth, optimized crystallizer electromagnetic stirring, dynamic light pressure, and secondary cooling capacity under low superheat conditions, comprehensively optimizing the process to shorten the time for supersaturated titanium and nitrogen elements in the molten steel to precipitate TiN during cooling, achieving C1.0 segregation at the slab center. After heating, rough rolling, and finishing rolling at a temperature of 850℃~880℃, rolling is performed within the austenitic region. This fully utilizes the high plasticity of austenite, while dynamic recrystallization refines the austenite grains. Rapid cooling in the pre-laminar flow cooling stage inhibits austenite grain growth. Rapidly cooling the high-temperature austenite to the coiling temperature range of 560℃~600℃ suppresses pearlite transformation. Under rapid cooling conditions, Ti and N atoms do not have sufficient time to diffuse and aggregate, thus limiting atomic migration and promoting the precipitation of TiN in a finer, more dispersed morphology. This yields a high-strength beam steel with excellent low-temperature toughness of 700MPa, yield strength ≥650MPa, tensile strength ≥760MPa, elongation ≥20%, and mechanical properties superior to national standards; grain size ≥12.5 grade; and low-temperature impact toughness ≥150J at -20℃ and ≥80J at -40℃. This patented design reduces costs by ≥200 RMB / ton compared to Nb and Ti microalloying designs. Comparative Example 1 and Examples 1-4: Table 1 shows the chemical composition of Comparative Example 1 and Examples 1-4 of the present invention.

[0045] Table 1 Chemical Composition

[0046] In Table 1, the comparative example is the traditional steel enterprise's 700MPa high-strength beam steel process. The Nb precipitation strengthening method replaces part of the titanium precipitation strengthening, which reduces the probability of TiN non-metallic inclusion precipitation and is beneficial to improving low-temperature impact toughness. However, the nitrogen content of 0.0053% increases the probability of TiN non-metallic inclusion precipitation. Overall, the precipitation of TiN non-metallic inclusions actually increases, and the overall low-temperature impact toughness is not improved. Moreover, the cost is too high.

[0047] Examples 1-4 are low-cost, high-strength beam steel processes with excellent low-temperature toughness of 700MPa. Nitrogen control and large-size TiN precipitation are achieved in four dimensions, which greatly improves low-temperature impact toughness while reducing costs: (1) Low-carbon converter smelting suppresses nitrogen increase; (2) Nitrogen control is achieved under LF refining at 0.2MPa micro-positive pressure; (3) Full-process protective casting of slabs enhances the cooling capacity of the secondary cooling zone, shortening the TiN precipitation time from 800 seconds to 630 seconds, effectively suppressing TiN precipitation; (4) Low coiling rapidly cools the high-temperature austenite, promoting TiN precipitation in a finer, more dispersed form.

[0048] Table 2 shows the key steelmaking process parameters of the comparative examples and Examples 1-4 of this invention.

[0049] Table 2 Key process parameters for steelmaking

[0050] In Table 2, the control example is the 700MPa high-strength beam steel process of a general steel enterprise. The lime + fluorite content is 8.0 kg / t, and the aluminum block / aluminum particle content is 2.0 kg / t. The slag volume is large and the cost is high. The ineffective slag material does not achieve the ultimate desulfurization effect, resulting in an LF endpoint S of 0.005%, which is relatively well controlled. The slag melting point is 1523℃, which can effectively adsorb non-metallic inclusions. Due to the excessive insertion depth of the nozzle, the meniscus shifts downward, which entrains the protective slag into the solidification front, forming large-sized non-metallic inclusions. The superheat of 26℃ is too high. Under the process control of a secondary cooling water ratio of 0.5 L / kg, a crystallizer electromagnetic stirring of 300A / 6Hz, and a dynamic light pressure reduction of 3.0 mm, the TiN non-metallic inclusions are severely agglomerated, and the central segregation of C2.0 is relatively good.

[0051] Examples 1-4 illustrate a low-cost, high-strength beam steel process with excellent low-temperature toughness (700MPa). The lime + fluorite content is 6.5kg / t-7.0kg / t, and the aluminum block / granule content is 1.5-2.0kg / t, reducing slag volume and lowering costs. Combined with calcium carbide (0.48kg / t-0.50kg / t) and silicon carbide (0.13kg / t-0.15kg / t) for composite deoxidation, the final LF endpoint S ≤ 0.002%, achieving optimal desulfurization. The slag melting point is 1450℃-1470℃, and the liquid slag better adsorbs non-metallic inclusions. The superheat is 16℃-19℃. Under process control with a secondary cooling water ratio of 0.7L / kg, crystallizer electromagnetic stirring at 350A / 7Hz, and a dynamic light pressure reduction of 4.2mm, the segregation of TiN non-metallic inclusions is greatly reduced, and the central segregation of C1.0 is good, providing excellent prerequisites for controlled cooling during steel rolling.

[0052] Table 3 shows the key rolling process parameters of the comparative examples and Examples 1-4 of this invention. Table 3 Key process parameters for steel rolling

[0053] In Table 3, the comparative example is the 700MPa high-strength beam steel process of a general steel enterprise. The roughing rolling temperature is 1080℃ and the finishing rolling temperature is 882℃. Front-end cooling is adopted with a cooling rate of 18.4℃ / s. The relatively low cooling rate can effectively suppress TiN precipitation. However, due to the high nitrogen content of 53ppm and the low specific water content of 0.5L / kg in the billet, the TiN non-metallic inclusions are more severely agglomerated, which reduces the low-temperature impact toughness to a certain extent. It can basically meet the requirements of 700MPa high-strength beam steel.

[0054] Examples 1-4 describe low-cost, high-strength beam steel processes with excellent low-temperature toughness (700MPa). The roughing and finishing rolling temperatures are 1075℃-1092℃, and the finishing rolling temperatures are 866℃-895℃, all within the austenitic region. This fully utilizes the high plasticity of austenite and refines austenite grains through dynamic recrystallization. Rapid cooling in the initial laminar flow stage inhibits austenite grain growth. The cooling rate of 22.4℃ / s-22.8℃ / s suppresses pearlite transformation. Under rapid cooling conditions, Ti and N atoms do not have sufficient time to diffuse and aggregate, thus limiting atomic migration and promoting the precipitation of TiN in a finer, more dispersed form. Furthermore, the low nitrogen content (41ppm-45ppm) and high water content (0.7L / kg) in the cast billet significantly reduce the segregation of TiN nonmetallic inclusions, resulting in significantly improved low-temperature impact toughness and better meeting the requirements of low-cost, high-strength beam steel processes with excellent low-temperature toughness (700MPa).

[0055] Table 4 shows the test results of the comparative example and Examples 1-4 of the present invention.

[0056] In Table 4, the comparative example is the 700MPa high-strength beam steel process of a general steel enterprise. As explained in Table 2, due to the excessive insertion depth of the sprue, the meniscus shifts downward, causing the protective slag to be drawn into the solidification front, forming large-sized non-metallic inclusions, resulting in millimeter-sized non-metallic inclusions. The remaining Class A and Class D non-metallic inclusions are due to the high melting point of the slag (1523℃) and the lack of extreme desulfurization. The cooling rate is 18.4℃ / s, which is relatively low, and the grain size is only 10.0 grade. The impact energy at -20℃ is 78J, and the impact energy at -40℃ is 35J. The 700MPa high-strength beam steel of the comparative example can basically meet the process requirements of non-cold regions, but does not meet the process requirements of cold regions.

[0057] Examples 1-4 describe a low-cost, high-strength beam steel process with excellent low-temperature toughness (700MPa). The final LF (sulfurization process) concentration (S) is ≤0.002%, achieving optimal desulfurization. Furthermore, the slag melting point of 1450℃-1470℃ allows for better adsorption of non-metallic inclusions in the liquid slag, resulting in excellent overall control of non-metallic inclusions. A cooling rate of 22.4℃ / s-22.8℃ / s inhibits pearlite transformation. Under rapid cooling conditions, Ti and N atoms do not have sufficient time to diffuse and aggregate, thus limiting atomic migration and promoting the precipitation of TiN in a finer, more dispersed morphology. Simultaneously, the high cooling rate increases the grain size to ≥12.5 grade. In summary, the impact energy at -20℃ is ≥150J, and at -40℃ it is ≥80J, making this low-cost, high-strength beam steel with excellent low-temperature toughness perfectly suited for use in extremely cold regions.

[0058] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0059] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing 700MPa high-strength beam steel with excellent low-temperature toughness for use in high-altitude and cold regions, characterized in that, include: S1. Select the low-carbon mode for converter smelting, strengthen the stirring of the molten pool by top and bottom blowing to suppress nitrogen increase, wash the slag after tapping, and form slag in advance. S2 and LF refining, nitrogen control under 0.2MPa micro-positive pressure, to create low melting point refining slag and extreme desulfurization, to remove large non-metallic inclusions, calcium treatment to modify residual MnS and Al2O3, to remove small non-metallic inclusions. S3. Slab continuous casting with full protection pouring, under the premise of low superheat, optimize the crystallizer electromagnetic stirring, dynamic light pressure, and cooling capacity of the secondary cooling zone to shorten the TiN precipitation time. S4. The heating, roughing and finishing processes are carried out in sequence. The finishing process is carried out in the austenite region. The low coiling rapidly cools the high-temperature austenite, which promotes the precipitation of TiN in a finer and more dispersed form.

2. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 1, characterized in that, S1 includes: The converter was turned over, resulting in a carbon content of 0.03% to 0.04%. By strengthening the stirring of the molten pool through top and bottom blowing, the carbon-oxygen product is no more than 0.0025%, and the dissolved oxygen content in the steel is 0.0625% to 0.0833%. CO bubbles and bottom-blown Ar bubbles form a local low-pressure zone, which reduces the nitrogen partial pressure and accelerates the diffusion and removal of nitrogen atoms into the bubbles. During the slag washing process, 0.1 kg / t to 0.2 kg / t of carbon raiser for pre-deoxidation, 2.5 kg / t to 3.0 kg / t of lime, 1.5 kg / t to 2.0 kg / t of aluminum blocks, and 0.15 kg / t to 0.25 kg / t of calcium carbide are added sequentially for deep deoxidation to ensure that the oxygen content is no more than 0.0020%.

3. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 1, characterized in that, S2 include: Molten steel is refined in the LF furnace, with the dust removal opening maintained at 5% to 10%. The LF refining process is carried out under a slightly positive pressure of 0.2 MPa. Calcium carbide and silicon carbide undergo composite deoxidation to generate CO, which forms a CO protective layer on the water-cooled furnace cover to reduce nitrogen accumulation. The total amount of lime in the converter and LF furnace is 6.0 kg / t to 7.0 kg / t, the amount of calcium carbide in the converter and LF furnace is 0.4 kg / t to 0.6 kg / t, the amount of silicon carbide in the LF furnace is 0.1 kg / t to 0.2 kg / t, and the amount of ferrosilicon powder in the LF furnace is 0.5 kg / t for composite diffusion deoxidation and extreme desulfurization. The amount of sulfur in the LF furnace is no more than 0.003%, which produces low-melting-point refining slag to adsorb large-sized non-metallic inclusions. The pure calcium line modifies the residual MnS and Al2O3 at a speed of 3.0 m / s to 4.0 m / s, and small-sized non-metallic inclusions are removed by the low-melting-point refining slag.

4. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 1, characterized in that, S3 include: Open the argon pipeline at the long nozzle, with an argon flow rate of 30NL / min to 100NL / min. The argon flow rate at the tundish nozzle is 2NL / min to 5NL / min. The argon flow rate for the stopper rod is 2NL / min to 5NL / min. The argon flow rate for the interplate is 2NL / min to 5NL / min. By improving the nozzle insertion depth to 150mm-160mm, optimizing the crystallizer electromagnetic stirring at 350A / 7Hz, dynamic light pressing down to 4.0mm-4.2mm, and the secondary cooling zone water volume to 0.6L / kg-0.8L / kg under the premise of superheated temperature of 10℃-25℃, the precipitation time of supersaturated titanium and nitrogen elements in the molten steel during cooling is shortened, and the slab center segregation reaches C1.

0.

5. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 1, characterized in that, S4 include: The finishing rolling temperature is 850℃~880℃. It utilizes the high plastic deformation capacity of austenite, while refining austenite grains through dynamic recrystallization and suppressing austenite grain growth through rapid cooling in the laminar flow cooling front section. By rapidly cooling the high-temperature austenite to the coiling temperature range of 560℃ to 600℃ at a cooling rate of 20℃ / s to 30℃ / s, the pearlite transformation is suppressed. Under rapid cooling conditions, Ti and N atoms do not have sufficient time to diffuse and aggregate, thus limiting atomic migration and promoting the precipitation of TiN in a finer and more dispersed form.

6. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 1, characterized in that, Also includes: S5. Through sampling and testing, the mechanical properties, grain size, and low-temperature toughness of the 700MPa high-strength beam steel were determined.

7. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 6, characterized in that, Mechanical properties include: yield strength, tensile strength, and elongation.

8. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 7, characterized in that, The yield strength of high-strength beam steel with low-temperature toughness of 700MPa is not less than 650MPa, the tensile strength is not less than 760MPa, and the elongation is not less than 20%.

9. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to claim 7, characterized in that, The grain size of the high-strength beam steel with a low-temperature toughness of 700MPa is not less than grade 12.5; the impact toughness at -20℃ is not less than 150J, and the impact toughness at -40℃ is not less than 80J.

10. The method for preparing 700MPa high-temperature toughness beam steel for high-altitude and cold regions according to any one of claims 1-9, characterized in that, The chemical composition of the high-strength beam steel with low-temperature toughness of 700MPa, by mass percentage, includes C: 0.04%~0.08%, Si≤0.20%, Mn: 1.40%~1.60%, P≤0.020%, S≤0.005%, Al0.02%~0.06%, Ti: 0.100%~0.150%, O≤0.002%, N≤0.0040%, with the balance being Fe and unavoidable impurities.

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