A method for refining the solidification structure of steel based on two-step nucleation

Abstract

The application discloses a molten steel solidification structure refining method based on two-step nucleation and belongs to the technical field of steel smelting. The application introduces the thought of changing harm into benefit into the field of steel smelting, converts the inclusions inevitably generated in a metallurgical process into endogenous nucleation cores meeting the requirements of two-step nucleation theory, and realizes solidification structure refinement.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, specifically involving grain refinement and control technology in the solidification process of molten steel. In particular, it is a technology based on advanced solidification theory that actively regulates the non-metallic inclusions that are inevitably generated in the metallurgical process to optimize the solidification structure, thereby improving the quality of steel. Background Technology

[0002] The grain size and morphology in the solidification structure of steel directly determine the macroscopic and microscopic structure of the billet and ingot, and have a decisive influence on the final mechanical properties (strength, toughness, plasticity), processing performance, and anisotropy of the steel. Obtaining a fine and uniform equiaxed grain structure has been a long-term goal pursued by steel metallurgists.

[0003] Currently, commonly used solidification microstructure refinement methods in the industry mainly include nucleating agent addition, dynamic grain refinement, and rapid cooling. Nucleating agent addition involves adding elements such as aluminum, titanium, and boron to molten steel to form foreign particles like Al2O3, TiN, TiC, and BN as heterogeneous nucleation sites. This method is the most widely used, but high-quality nucleating agents (such as titanium) are expensive, and improper control of the addition amount can easily lead to the formation of large-sized clusters of brittle inclusions, impairing the fatigue life and impact toughness of the steel. The size, distribution, and phase composition of the nucleation sites are greatly affected by factors such as steel composition, temperature, and stirring, resulting in poor reproducibility. Dynamic grain refinement uses electromagnetic stirring, mechanical vibration, or ultrasonic treatment to apply external force to disturb dendrites, causing them to break down and become new nuclei. This method requires large equipment investments and consumes a lot of energy. For castings with a cross-sectional diameter greater than 600mm, the skin effect makes it difficult for energy to penetrate to the core of the casting, and the refinement effect significantly decreases from the surface to the core. Rapid cooling increases the undercooling by increasing the cooling rate to improve the nucleation rate, but it is uneconomical for large-scale continuous casting or ingot casting due to equipment capacity limitations, and it can easily lead to increased casting stress and crack defects.

[0004] The methods described above all belong to the "external intervention" model, which involves adding energy or materials from outside the system to achieve refinement. This model fails to effectively utilize the internal resources of the iron and steel metallurgical system. During steelmaking, a large amount of resources are consumed for deoxidation, desulfurization, and alloying, generating non-metallic inclusions such as oxides, sulfides, and nitrides. Traditionally, these inclusions are considered harmful "metallurgical consumables" that need to be removed through refining processes such as slag washing, argon blowing, and filtration. This process incurs costs both by adding nucleating agents and by removing endogenous particles with nucleation potential, creating a technological paradox and a cost trap. Summary of the Invention

[0005] To address the problems in the existing technology, this invention provides a method for refining the solidification structure of molten steel based on two-step nucleation. This method introduces the idea of ​​"turning adversity into advantage" into the field of iron and steel metallurgy. By transforming the inclusions that are inevitably generated during the metallurgical process into endogenous nucleation cores that meet the requirements of the two-step nucleation theory, the solidification structure is refined, resulting in a finer and more uniform solidification structure than that obtained by traditional methods.

[0006] The technical solution adopted in this invention is as follows: This invention provides a method for refining the solidification structure of molten steel based on a two-step nucleation process to reduce metallurgical consumption, comprising the following steps: Step 1: Screening of target nuclei Based on the composition of the target steel, target inclusions that can serve as effective nucleation substrates are screened out through physicochemical properties, thermodynamic stability, and crystallographic compatibility; wherein the physicochemical properties include solidification properties and wettability.

[0007] The target steel grades include, but are not limited to: alloy structural steel (such as 42CrMo, 40Cr, 40CrNiMo, 20CrMnTi), bearing steel (such as GCr15), spring steel (such as 55SiCr, 60Si2Mn), tool steel (such as T8, T10, Cr12MoV), pipeline steel (such as X70), and medium-carbon alloy tool steel (carbon content of 0.3-0.7%). During the metallurgical process, these steel grades inevitably produce oxide, sulfide, or nitride inclusions. Through the two-step nucleation theory design and precise metallurgical control of this invention, these inclusions can be transformed in situ into effective nucleation nuclei.

[0008] The solidification characteristics include the solidification process and the crystal lattice of the resulting iron matrix. The solidification process is peritectic reaction solidification or layer-by-layer solidification, and the crystal lattice of the resulting iron matrix is... or .

[0009] The wettability refers to the good wettability between the target inclusion and the molten steel, and the low interfacial energy. The quantitative index of wettability is the contact angle (θ). If the contact angle (θ) is less than 90°, the wettability between the inclusion and the molten steel is good and the interfacial energy is low. The smaller θ is, the better the wettability.

[0010] Thermodynamic stability: The target inclusion can exist stably at the steel molten casting temperature (1500-1650℃, preferably 1550-1620℃); by calculating the Gibbs free energy ΔG of the target inclusion at the steel molten casting temperature, ΔG<0 indicates that the inclusion can exist stably without decomposition or dissolution.

[0011] The thermodynamic stability calculation uses thermodynamic calculation software and its supporting database. The full composition information of the target steel grade (including the mass fraction of elements such as C, Si, Mn, P, S, Al, Cr, Mo, Ti, Nb, V, N, and O) is input, and the calculation is performed based on the principle of minimizing Gibbs free energy.

[0012] The crystallographic matching: the target inclusion and the iron matrix ( or They have a low lattice mismatch ( ), <12%, preferably δ<6% (highly efficient nucleating agent precursor). Based on thermodynamic calculations confirming the stable formation of the target inclusion, the lattice mismatch between the inclusion and the iron matrix is ​​calculated using the formula δ=|d(hkl). n -d(hkl) s | / d(hkl) s ×100%, where d(hkl) n d(hkl) represents the interplanar spacing of the nucleation core. s This refers to the interplanar spacing of the iron matrix. For example, inclusions. Spinel and The mismatch of the (100) plane is approximately 3.1%, and the inclusion TiN and The mismatch of the (100) facet is approximately 3.9%.

[0013] The target inclusions that can serve as effective nucleation substrates are selected as one or more of oxides, rare earth oxides and nitrides; among which, the oxide is magnesium aluminum spinel ( Rare earth oxides and sulfides are , One or two of the nitrides are TiN and ZrN.

[0014] Step 2: Selection of nucleating agent precursor Based on the target inclusions determined in step one, an additive capable of reacting with existing elements in the molten steel to generate the target phase is selected as a nucleating agent precursor.

[0015] For targets containing oxide inclusions: This method is suitable for aluminum-deoxidized steel with an [Als] content of 0.015%-0.050%. After Al deoxidation, the molten steel contains a large amount of flocculent Al2O3. Metallic Mg or Mg alloys (e.g., Si-Mg alloys, Ni-Mg alloys) are selected as precursors for magnesium treatment. The reaction is as follows: The generated MgO will immediately combine with Al2O3 to form .

[0016] For target inclusions of rare earth oxides and sulfides: suitable for steel grades with oxygen content of 5-30 ppm and sulfur content of 0.005%-0.02%, rare earth (RE) alloys (such as Ce-Fe alloys or Ce-La alloys) are selected as precursors for rare earth treatment. Taking Ce as an example, its reaction is: x[Ce]+y[O]+z[S]=CexOySz(s). By controlling the [O] / [S] activity ratio (0.53-0.96), Ce2O2S can be preferentially generated.

[0017] For nitrides as the target inclusions: When the molten steel contains a certain amount of nitrogen (40-120ppm, preferably 60-80ppm), Ti or Zr, a strong nitride-forming element, can be added to generate fine TiN or ZrN particles. High-titanium iron (Ti content ≥70%) or zirconium iron alloy can be selected as the precursor for Ti or Zr treatment.

[0018] Step 3: Precise metallurgical process control This step is crucial to ensuring technical feasibility, and the following process parameters must be precisely controlled: 1) Timing of nucleating agent precursor addition: Choose the later stage of refining, the later stage of LF refining or RH vacuum treatment (RH refining), preferably 5-15 minutes before the end of refining, to ensure that the composition and temperature of the molten steel are stable and that large inclusions have enough time to float to the surface.

[0019] 2) Amount of nucleating agent precursor added: The principle is to ensure sufficient conversion into harmful inclusions (such as modifying Al2O8 clusters) while avoiding excessive addition that could generate a single harmful phase (such as excessive Mg forming pure MgO). The amount added is determined according to the formula: The formula for calculating the amount of magnesium added is: Mg addition (kg / t) = k × [Als]% × 1000 + Δ, where [Als]% is the acid-soluble aluminum content in the steel, k is the proportionality coefficient (0.3-0.8), and Δ is the excess coefficient (0.2-0.5 kg / t). The amount of Mg added is 0.5-1.5 kg / ton of steel.

[0020] The formula for calculating the amount of rare earth added is: Rare earth addition (kg / t) = ([O] × M RE / M O +[S]×M RE / M S )×η×1000, where [O] and [S] are the mass fractions of oxygen and sulfur in the molten steel, and M RE M O M S The molar masses of rare earth elements, oxygen, and sulfur are respectively, and η is the yield coefficient, ranging from 1.5 to 3.0. The rare earth element addition is 0.3-1.0 kg / ton of steel, and the residual rare earth element content in the steel is controlled to be 0.010%-0.030%.

[0021] The formula for calculating the amount of high-titanium iron added is: TiFe addition amount (kg / t) = ([Ti]s×1000) / (ω Ti ×η Ti ), where [Ti]s is the target acid-soluble titanium content, ω Ti η represents the mass fraction of Ti in high-titanium iron. Ti The yield of titanium is given. The amount of high-titanium ferrophosphate added is 0.3-1.2 kg / ton of steel, controlling the acid-soluble titanium [Ti]s content in the steel to be 0.015%-0.025%. With [Ti]s = 0.020%, ω... Ti =70%, η Ti Taking 72% as an example, the amount of high-titanium iron added is 0.40 kg / ton of steel.

[0022] The formula for calculating the amount of zirconium iron added is: ZrFe addition amount (kg / t) = ([Zr] × 1000) / (ω) Zr ×η Zr ), where [Zr] is the mass fraction of the target residual zirconium content, ω Zr η is the mass fraction of Zr in the zirconium-iron alloy. Zr The yield of zirconium is given. The amount of ferrozirconium added is 0.1-2.5 kg / ton of steel, controlling the residual zirconium content in the steel to be 0.005%-0.030%. The target residual zirconium content [Zr] is 0.015%, and the Zr content ω in the ferrozirconium alloy is... Zr =40%, zirconium yield η Zr Taking 55% as an example, the amount of zirconium added is 0.68 kg / ton of steel.

[0023] 3) Method of adding nucleating agent precursor: For Mg metal treatment, the nucleating agent precursor is added using a wire feeding method with a core wire diameter of 9-13 mm and a feeding speed of 8-20 m / min. This method allows the additive to quickly and uniformly penetrate deep into the molten steel, resulting in rapid reaction, high yield, and easy automation control. For Mg alloy treatment, rare earth treatment, and Ti or Zr treatment, the nucleating agent precursor is added in block or powder form.

[0024] 4) Temperature and stirring during nucleating agent precursor addition: Control the molten steel temperature within a reasonable range of 1500-1650℃ (preferably 1550-1620℃). Before, during, and after addition, use appropriate bottom-blowing argon stirring (before adding the nucleating agent precursor, the bottom-blowing argon flow rate is 80-150 NL / min, stirring for 3-5 minutes; during the addition of the precursor, the bottom-blowing argon flow rate is 50-100 NL / min; after adding the precursor, the bottom-blowing argon flow rate is reduced to 15-30 NL / min, and soft blowing is performed for 8-15 minutes) to ensure sufficient reaction, uniform composition, and promote the dispersion of fine particles generated by the reaction, while allowing larger particles that cannot be refined to float to the slag.

[0025] Step 4: Solidification and Shaping After completing the above treatment, casting should be carried out as soon as possible. The casting method is ingot casting or continuous casting (including slab continuous casting and billet continuous casting). During the solidification process of molten steel, these fine inclusions, which we actively manufactured and controlled, are dispersed and act as a highly efficient nucleation substrate, significantly reducing the nucleation work and greatly increasing the nucleation rate, thereby obtaining a fine and uniform equiaxed crystal structure throughout the entire cross-section.

[0026] Furthermore, during Ti treatment, the time from the addition of Ti to the start of continuous casting solidification must be strictly controlled within 15-25 minutes.

[0027] Furthermore, for target inclusions that are nitrides: if the [N] content in the molten steel during refining is found to be below the lower limit of the target range, a nitrogen addition operation is required, using manganese nitride as the nitrogen source. When it is determined that nitrogen addition is necessary, the precise amount of manganese nitride to be added is calculated based on the difference between the measured [N] value and the target value using the following formula, and then added to the molten steel: Manganese nitride addition amount (kg / t) = (target [N] content - measured [N] content) × 1000 / (N content in manganese nitride × yield) Adding manganese nitride requires stirring the molten steel with argon gas. During the addition process, the bottom-blowing argon gas flow rate should be controlled at 50-100 NL / min, and stirring should be performed for 3-5 minutes to promote rapid melting and uniform dispersion of the manganese nitride. After adding manganese nitride, the bottom-blowing argon gas flow rate should be reduced to 15-30 NL / min, and soft blowing should be performed for 5-10 minutes to ensure that the added nitrogen element is evenly distributed in the molten steel and to promote the flotation of inclusions.

[0028] The core idea of ​​this invention is active design and in-situ manufacturing. Instead of passively accepting the random generation of inclusions, it actively designs target inclusion phases that are stable in molten steel and well matched with the iron lattice based on the two-step nucleation theory. Then, through precise metallurgical operations, it uses existing elements in the molten steel (such as [O], [S], [N]) as reactants to generate these target inclusions in situ.

[0029] Compared with the prior art, the present invention has the following significant advantages: (1) This invention adds nucleating agent precursors to molten steel in the later stage of refining, and uses the existing oxygen, sulfur and nitrogen elements in the molten steel to generate fine oxides, rare earth oxygen sulfides or nitrides as nucleation cores. During solidification, two-step nucleation is triggered, transforming the "metallurgical consumables" that need to be removed in the traditional concept into valuable nucleation resources. From the perspective of working principle, it realizes the refinement of solidification structure by "turning harm into benefit".

[0030] (2) Compared with the traditional process, in Example 1, the process of the present invention increases the proportion of equiaxed grain regions in 42CrMo steel from 32% to 75%, refines the average grain size from 315μm to 180μm (a refinement of 43%), and reduces the center shrinkage porosity rating from 2.0 to 1.0. In Example 2, the process of the present invention increases the proportion of equiaxed grain regions in X70 pipeline steel from less than 20% to over 75%, refines the austenite grain size by about 40%, and improves the center segregation rating from Class C 1.5 to Class B 1.0. Through comparison, it is shown that all indicators of the process of the present invention are significantly better than those of the traditional process, directly proving the technical superiority of the present invention.

[0031] (3) The lattice mismatches of the target inclusions MgO·Al2O3, Ce2O2S and TiN selected in this invention with δ-Fe or γ-Fe are 3.1%, 3.9% and 3.9% respectively (all less than 6%), and the contact angles are all less than 90°. The low lattice mismatch and good wettability synergistically reduce the nucleation energy barrier, increasing the nucleation core number density from 15 / mm² in the traditional process to 100 / mm² (an increase of 7 times), thereby significantly improving the nucleation rate in the early stage of solidification and obtaining a fine and uniform equiaxed crystal structure. Attached Figure Description

[0032] Figure 1 This is a process flow diagram of the present invention.

[0033] Figure 2 This is a statistical comparison histogram of the grain size distribution between the steel solidification microstructure refinement method based on two-step nucleation and the traditional method used in Embodiment 2 of the present invention. Detailed Implementation

[0034] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0035] This invention is a method for refining the solidification structure of molten steel based on a two-step nucleation process to reduce metallurgical consumption. The invention is illustrated in detail with three examples from a steel plant.

[0036] Example 1: Application in alloy structural steel 42CrMo (using Mg treatment to modify Al2O3 inclusions) 42CrMo is widely used in the manufacture of key components such as high-strength connecting rods, spindles, and gears. This steel undergoes final deoxidation with aluminum, which generates a large number of high-melting-point clusters of Al2O3 inclusions. These inclusions not only impair fatigue performance and toughness, but also act as inefficient nucleation sites. Furthermore, their aggregation and flotation cause problems with the cleanliness of the molten steel, representing a typical metallurgical waste. The as-cast microstructure obtained by traditional processes exhibits well-developed columnar crystals, leading to macroscopic segregation and anisotropy in mechanical properties.

[0037] Step 1: Theoretical Design and Objective Determination Based on the composition of the target steel, target inclusions that can serve as effective nucleation substrates are screened out through physicochemical properties, thermodynamic stability, and crystallographic compatibility; wherein the physicochemical properties include solidification properties and wettability.

[0038] The typical composition of 42CrMo steel, by mass percentage, is C: 0.42%, Si: 0.25%, Mn: 0.65%, Cr: 1.0%, Mo: 0.2%, S≤0.035%, P≤0.035%, with Fe as the balance. It exhibits a layer-by-layer solidification characteristic, with γ-Fe as the primary phase.

[0039] Thermodynamic calculations show that Al₂O₃ is a stable phase at the refining and casting temperatures. Al₂O₃ has a hexagonal crystal system and a lattice mismatch of 16% with γ-Fe, exceeding the 12% threshold, thus constituting an ineffective nucleating agent. Magnesium aluminum spinel has a ΔG of -49913.5 J / mol < 0. Magnesium aluminum spinel was selected (…). ) is the target phase, which also has a face-centered cubic structure and has a lattice mismatch of 3.1% with the (100) plane of γ-Fe, making it a highly efficient nucleating agent. The contact angle with molten steel is 65°, θ<90°, indicating good wettability.

[0040] Step 2: Selection of nucleating agent precursor Pure magnesium cored wire with a diameter of 9mm is used.

[0041] Step 3: Precision metallurgical process control Process route: Electric furnace (EAF) smelting → LF furnace refining and alloying → Mg treatment → soft argon blowing → die casting into 5-ton steel ingots.

[0042] The timing and conditions for adding pure magnesium cored wire are as follows: During the LF furnace refining process, aluminum granules are first added for final deoxidation, reducing the dissolved oxygen in the molten steel to below 5 ppm and adjusting the acid-soluble aluminum [Als] to 0.035%. Then, Cr and Mo alloying elements are added to adjust the molten steel composition to 42CrMo, and the temperature is raised to 1582℃. A spectral sample is taken to confirm that the composition is qualified. The bottom-blown argon flow rate is adjusted to 80 NL / min, and stirring is performed for 5 minutes. Ten minutes before the end of refining, pure magnesium cored wire is fed in; the feeding speed is 12 m / min, which ensures sufficient penetration depth of the core wire without causing violent splashing of magnesium vapor due to excessive speed. The feeding amount is 0.95 kg / ton of steel. This value is calculated based on the acid-soluble aluminum content ([Als] = 0.035%) in the steel using the following empirical formula: Mg addition amount (kg / t) = 0.5 [Als]% 1000 + 0.3 (excess coefficient); This dosage aims to modify most of the Al2O3 while generating a small amount of MgO to promote spinelization, but avoiding the formation of a single MgO phase. Maintain a weak stirring state with a bottom-blown argon flow rate of 70 NL / min during wire feeding.

[0043] Soft argon blowing: After the wire feeding is completed, reduce the bottom-blowing argon flow rate to 20 NL / min and perform soft argon blowing for 10 minutes to ensure the fine particles generated by the reaction are absorbed. The particles are evenly dispersed in the molten steel, while large inclusions that have not been modified or have grown due to collisions float to the slag.

[0044] Scanning electron microscopy analysis of the steel ingots revealed that, in the traditional process (where 42CrMo steel from the same furnace is refined and alloyed in the LF furnace without Mg treatment, and then directly cast into 5-ton ingots after 5 minutes of soft argon blowing, with other process conditions identical to the present invention), inclusions were mainly larger than 20 mm. Irregular Al3O2 clusters, with a number density of approximately 15 clusters / mm². 2 In this embodiment of the invention, almost all process inclusions are transformed to a particle size of 1.0-3.0. Spheres or regular polygons Particle number density significantly increased to 100 particles / mm 2 This means that the number of effective nucleation sites per unit area has increased by nearly 7 times.

[0045] Traditional process steel ingots (from the same batch of 42CrMo molten steel, after refining and alloying in the LF furnace, are directly cast into 5-ton ingots after soft argon blowing for 5 minutes without Mg treatment; other process conditions are the same as the process of this invention) show well-developed columnar crystals in their longitudinal section, with the equiaxed crystal region accounting for only 32%. The average grain size, measured by the truncation method, is 315. The central shrinkage porosity rating is 2.0. In this embodiment of the invention, the entire cross-section of the steel ingot exhibits uniform, fine equiaxed grains, with the equiaxed grain region accounting for as much as 75%. The average grain size is refined to 180. The level of detail reached 43%. The central transpiration rate was 1.0.

[0046] Example 2: Application in high-nitrogen microalloyed oil pipeline steel X70 (using TiN to treat nitride inclusions) Pipeline steel X70 employs Nb-V-Ti microalloying and controlled rolling with air cooling. Improper control of nitrogen (N) in the steel can lead to the formation of coarse nitride deposits with V and Al, impairing toughness and weld heat-affected zone properties. This invention controls the inherent [N] in the steel to prevent the formation of VN or AlN, instead transforming it into a catalyst that promotes [solidification] in the early stages of solidification. Tiny, dispersed TiN particles that are undergoing nucleation.

[0047] Step 1: Theoretical Design and Objective Determination Based on the composition of the target steel, target inclusions that can serve as effective nucleation substrates are screened out through physicochemical properties, thermodynamic stability, and crystallographic compatibility; wherein the physicochemical properties include solidification properties and wettability.

[0048] The chemical composition of X70 pipeline steel, by mass percentage, is: C 0.11%, Si 0.26%, Mn 1.60%, P 0.008%, S 0.003%, Cr 0.22%, Ni 0.10%, Mo 0.08%, Nb 0.045%, V 0.025%, Ti 0.020%, N 0.008% (80ppm), with Fe as the balance.

[0049] Thermodynamic calculations show that at solidification temperatures above 1500°C, the formation free energy of TiN is much lower than that of VN and AlN. TiNΔG = -114823 J / mol < 0, indicating that TiN can spontaneously precipitate at the solidification temperature of molten steel. TiN has a face-centered cubic structure, similar to... The (100) plane lattice mismatch is 3.9%, classifying it as a highly efficient nucleating agent. The contact angle between TiN and molten steel is 55°, θ < 90°, indicating good wettability. Our goal is to generate a large number of nanoscale TiN particles in situ before the molten steel solidifies.

[0050] Step 2: Selection of nucleating agent precursor High-titanium iron alloy (Ti content 72%) was selected as the precursor.

[0051] Step 3: Precision metallurgical process control Process route: converter smelting → LF furnace refining → RH vacuum degassing → TiN treatment → soft argon blowing → slab continuous casting.

[0052] The key control steps are as follows: Step 1 (Deoxidation and Nitrogen Fixation): Aluminum is first added to the LF furnace for final deoxidation, reducing [O] to below 5 ppm. This prevents Ti from preferentially combining with O to form TiO2, which would consume available Ti.

[0053] Step 2 (Vacuum Degassing and Composition Fine-tuning): The steel undergoes RH vacuum degassing treatment, with deep removal of hydrogen (H) at a vacuum of 100 Pa. After degassing, the H content is 1.0-2.0 ppm. Nitrogen (N) is also removed, with the N content controlled at 80 ppm. Spectroscopic analysis results are obtained after the RH vacuum degassing treatment, and the composition is found to be within acceptable limits.

[0054] Step 3 (In-situ TiN Generation): After RH vacuum decarburization, aluminum is added for final deoxidation. After circulating for 3-5 minutes, when the molten steel temperature drops to 1560°C, high-titanium ferroalloy blocks are added through an alloy chute. Argon is continuously used as the driving gas for molten steel circulation and stirring throughout the RH treatment. During the degassing stage (before adding the high-titanium ferroalloy), the circulating argon flow rate is controlled at 100 Nm³ / h; during the addition of the high-titanium ferroalloy, the circulating argon flow rate is adjusted to 80 Nm³ / h. After adding the high-titanium ferroalloy, soft blowing argon is performed at a bottom blowing argon flow rate of 20 NL / min for 5 minutes to ensure uniform dissolution of the ferroalloy and sufficient reaction with TiN. The time from the addition of Ti to the start of continuous casting solidification is controlled within 15-25 minutes to prevent excessive Ostwald ripening and growth of TiN particles.

[0055] High-titanium ferroalloy addition amount: calculated based on the target acid-soluble titanium [Ti]s content (0.015%-0.025%) by dry weight and the titanium yield. In this example, [Ti]s is 0.02%, the Ti / N ratio is 3.5, and the yield is 70%. High-titanium ferroalloy addition amount = 0.02% × 1000 / (70% × 72%) = 0.4 kg / ton of steel.

[0056] In steel, [N] is mainly dissolved in the matrix or precipitates as VN / AlN later. The as-cast microstructure lacks effective nucleation sites. In this embodiment of the invention, samples were cut from continuously cast slabs using carbon extraction replication technology. Observation under a transmission electron microscope (TEM) revealed a large number of 5-50 nm TiN cubic particles uniformly dispersed in the as-cast microstructure. These particles are the nuclei that promote δ-Fe nucleation in the early stages of solidification.

[0057] Traditional continuous casting slabs (made from the same batch of X70 steel, refined in the LF furnace and degassed in the RH vacuum furnace without TiN treatment, with other process conditions the same as in this embodiment, directly cast into slabs) exhibit obvious coarse columnar crystals and a central segregation band in the center, with the central equiaxed crystal region accounting for less than 20%, and a central segregation rating of C-class 1.5. The slab produced by the embodiment of this invention has a uniform and fine microstructure throughout its thickness, with the central equiaxed crystal region accounting for over 75%, and a central segregation rating of B-class 1.0. Measurement of the original austenite grain size shows that after homogenization (heated to 1150℃ and held for 3 hours), the austenite grain size of the as-cast microstructure produced by the embodiment of this invention is approximately 40% finer than that of the traditional process.

[0058] from Figure 2 It can be seen that the grain size distribution of the traditional process exhibits a clear right-skewed characteristic, with the peak appearing in the 150-200 μm range. The distribution is wide and dispersed, and coarse grains larger than 400 μm exist, indicating that the solidified structure obtained by the traditional process has coarse and uneven grains. The grain size distribution of the process in this embodiment of the invention exhibits a sharp left-skewed characteristic, with the peak appearing in the 50-100 μm range. The distribution is narrow and concentrated, with no coarse grains larger than 400 μm. The average grain size is refined from 315 μm in the traditional process to 150-180 μm, a grain refinement of approximately 40%. The comparison shows that the present invention significantly improves the nucleation rate during solidification by generating a large number of fine and dispersed TiN nucleation cores in situ in the molten steel, causing the overall grain size to shift towards a finer direction, while also greatly improving the uniformity of the microstructure.

[0059] Example 3 The difference between Example 3 and Example 2 lies in the second step (vacuum degassing and composition fine-tuning): [N] removal. If the [N] content in the molten steel is below 80 ppm, manganese nitride (MnN) is added to the ladle via a soft-blowing argon station for precise replenishment. Manganese nitride is added to the ladle through an alloy chute or hopper. During the degassing stage (before adding manganese nitride), the circulating argon flow rate is controlled at 100 Nm³ / h. During the addition of manganese nitride, the bottom-blowing argon flow rate is controlled at 80 NL / min, and stirring is performed for 3-5 minutes. After adding manganese nitride, soft-blowing argon stirring is performed for 5 minutes (bottom-blowing argon flow rate 20 NL / min) to ensure uniform distribution of the added nitrogen in the molten steel. Finally, a second sample is taken for analysis to confirm that the final [N] content has accurately reached the target range.

[0060] Manganese nitride addition amount: Taking a target [N] of 80 ppm and a current [N] of 50 ppm as an example, and manganese nitride (containing 6.5% N), approximately 30 ppm of [N] needs to be added. Therefore, the manganese nitride addition amount = 30 × 10⁻⁶ ppm. -6 ×1000 / (0.065×0.75)=0.62 kg / ton of steel.

Claims

1. A method for refining the solidification microstructure of molten steel based on a two-step nucleation process, characterized in that, Includes the following steps: Step 1: Screening of target nuclei Based on the composition of the target steel, target inclusions that can serve as effective nucleation substrates are screened out through physicochemical properties, thermodynamic stability, and crystallographic compatibility; wherein, the physicochemical properties include solidification properties and wettability; Step 2: Selection of nucleating agent precursor Based on the target inclusions determined in step one, select additives that can react with existing elements in the molten steel to generate the target phase as nucleating agent precursors. Step 3: Metallurgical Process Control In the later stages of refining, nucleating agent precursors are added; Step 4: Solidification and Shaping After completing step three, the casting process begins.

2. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 1, characterized in that, In step one, the solidification characteristics include the solidification process and the crystal lattice of the resulting iron matrix. The solidification process is peritectic reaction solidification or layer-by-layer solidification, and the crystal lattice of the resulting iron matrix is... or ; The wettability is described as follows: the target inclusion has good wettability with the molten steel; Thermodynamic stability: The target inclusion can exist stably at the steel molten casting temperature; The crystallographic matching is described as follows: the target inclusion has a low lattice mismatch with the iron matrix.

3. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 2, characterized in that, In step one, the quantitative index of wettability is the contact angle. If the contact angle is <90°, the inclusions and molten steel have good wettability. The lattice mismatch is <12%, indicating a low lattice mismatch between the target inclusion and the iron matrix.

4. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 1, characterized in that, In step one, target inclusions that can serve as effective nucleation substrates are selected as one or more of oxides, rare earth oxysulfides, and nitrides; among which, the oxides are magnesium aluminum spinel, and the rare earth oxysulfides are... , One or two of the following, wherein the nitride is one or two of TiN and ZrN; In step two, for targets containing oxides: this is suitable for aluminum deoxidized steel with an [Als] content of 0.015%-0.050%, using metallic Mg or Mg alloys as precursors for magnesium treatment; wherein the Mg alloy is a Si-Mg alloy or a Ni-Mg alloy. For targets containing rare earth oxysulfides: suitable for steel grades with oxygen content of 5-30ppm and sulfur content of 0.005%-0.02%, rare earth alloys are selected as precursors for rare earth treatment; wherein, the rare earth alloys are Ce-Fe alloys or Ce-La alloys. For nitride inclusions: when the molten steel contains 40-120 ppm, high-titanium ferroalloy or zirconium ferroalloy is selected as the precursor and Ti or Zr treatment is performed; among them, the Ti content in high-titanium ferroalloy is ≥70%.

5. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 1, characterized in that, In step three, the later stage of refining is either LF refining or RH refining. Temperature and stirring of nucleating agent precursor addition: Control the temperature of molten steel within the range of 1500-1650℃; before, during, and after adding nucleating agent precursor, coordinate with bottom blowing argon stirring; In step four, the casting method is either ingot casting or continuous casting.

6. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 4, characterized in that, The formula for calculating the amount of magnesium added is: Mg addition (kg / t) = k × [Als]% × 1000 + Δ, where [Als]% is the acid-soluble aluminum content in the steel, k is the proportionality coefficient of 0.3-0.8, and Δ is the excess coefficient of 0.2-0.5 kg / t; The formula for calculating the amount of rare earth added is: Rare earth addition (kg / t) = ([O] × M RE / M O +[S]×M RE / M S )×η×1000, where [O] and [S] are the mass fractions of oxygen and sulfur in the molten steel, and M RE M O M S The molar masses of rare earth elements, oxygen, and sulfur are respectively, and η is the yield coefficient, ranging from 1.5 to 3.

0. The formula for calculating the amount of high-titanium iron added is: TiFe addition amount (kg / t) = ([Ti]s×1000) / (ω Ti ×η Ti ), where [Ti]s is the target acid-soluble titanium content, ω Ti η represents the mass fraction of Ti in high-titanium iron. Ti The yield of titanium; The formula for calculating the amount of zirconium iron added is: ZrFe addition amount (kg / t) = ([Zr] × 1000) / (ω) Zr ×η Zr ), where [Zr] is the mass fraction of the target residual zirconium content, ω Zr η is the mass fraction of Zr in the zirconium-iron alloy. Zr The yield of zirconium is given.

7. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 4 or 6, characterized in that, The amount of Mg added is 0.5-1.5 kg / ton of steel; The rare earth content is 0.3-1.0 kg / ton of steel; The amount of high-titanium iron added is 0.3-1.2 kg / ton of steel; The amount of ferrozirconium added is 0.1-2.5 kg / ton of steel.

8. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 4, characterized in that, When treating Mg metal, the nucleating agent precursor is added using the wire feeding method, with a core wire diameter of 9-13 mm and a wire feeding speed of 8-20 m / min. When treating Mg alloys, rare earths, Ti, or Zr, the nucleating agent precursor is added in block or powder form. When treating Ti, the time from the addition of Ti to the start of continuous casting solidification must be strictly controlled within 15-25 minutes.

9. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 5, characterized in that, Before adding the nucleating agent precursor, the bottom-blown argon flow rate is 80-150 NL / min, and the mixture is stirred for 3-5 minutes; during the addition of the nucleating agent precursor, the bottom-blown argon flow rate is 50-100 NL / min; after adding the nucleating agent precursor, the bottom-blown argon flow rate is reduced to 15-30 NL / min, and the mixture is gently blown for 8-15 minutes.

10. The method for refining the solidification microstructure of molten steel based on two-step nucleation according to claim 4, characterized in that, For the target inclusions being nitrides: if the [N] content in the molten steel during refining is found to be below 40 ppm, a nitrogen enrichment operation is required, and the nitrogen source used is manganese nitride; The amount of manganese nitride to be added is calculated using the following formula, and then added to the molten steel: Manganese nitride addition amount (kg / t) = (target [N] content - measured [N] content) × 1000 / (N content in manganese nitride × yield).

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