A casting method for improving the solidification quality of a large ingot

Abstract

The present application relates to a kind of methods for improving the solidification quality of large ingot, including smelting and pretreatment: the superheat of liquid steel is 100-120 ℃;Cold core system configuration: arrange cold core in mould, several cold cores are preheated to 245-255 ℃ After fixed in the bottom of mould, and be arranged in regular shape array;Dynamic layered casting control: carry out casting in multiple layers, spray Fe powder or Fe+Ce powder between each layer, odd layer sprays Fe powder, even layer sprays Fe+Ce powder;Liquid steel flow rate control is 17.5-18.5 kg / s, total casting time control is 6.5-7.5 minutes;Using multi-field coupling solidification.The present application aims to break through the technical contradiction of "size increases-quality declines" of large ingot, provide a kind of casting method for improving the solidification quality of large ingot, improve solidification structure homogenization, improve defect inhibition and performance optimization, improve the flaw detection qualified rate of large ingot.

Description

Technical Field

[0001] This invention relates to the field of metallurgical casting technology, and in particular to a casting method for improving the solidification quality of large ingots. Background Technology

[0002] Large ingots are core raw materials for critical equipment such as nuclear power plant rotors and marine crankshafts, and their solidification quality directly determines the service performance of the final products. Currently, the industry faces three major technological bottlenecks:

[0003] 1. Difficulty in controlling solidification structure: In traditional processes, the proportion of columnar crystals in ingots weighing over 200 tons generally exceeds 70%, leading to significant anisotropy in mechanical properties. The macrosegregation index (GSI) is as high as 0.4-0.6, and the fluctuation range of key elements (such as C and Mn) reaches ±0.15%, severely restricting the homogeneity of forgings. A more prominent problem is that the solidification time of large ingots generally exceeds 100 hours, with cooling rates in different regions differing by more than 100 times, forming severe microstructure gradients.

[0004] Hard inclusions, typically larger than 20 μm (Class B rating ≥ 2.5), become preferred initiation points for fatigue cracks. Furthermore, due to solidification stress concentration, the hot crack susceptibility coefficient (HCS) often exceeds the critical value of 0.35, resulting in a persistently high scrap rate.

[0005] (℃), which can easily induce microcracks at the interface. The industry urgently needs an innovative solution that can simultaneously address the issues of structure, defects, and energy consumption.

[0006] Therefore, there is an urgent need to invent a process that can improve the solidification quality of large ingots. Summary of the Invention

[0007] This invention aims to overcome the technical contradiction of "increased size - decreased quality" in large ingots, and provides a casting method to improve the solidification quality of large ingots, enhance the homogenization of solidification structure, improve defect suppression and performance optimization, and increase the pass rate of flaw detection in large ingots.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for improving the solidification quality of large ingots, employing a multi-point internal cold core combined with composite layered casting, specifically includes the following steps:

[0010] 1) Smelting and Pretreatment: Smelting is carried out in a vacuum induction furnace, with the vacuum level controlled at ≤10Pa and the melting temperature at 1580~1600℃. The raw material ratio for molten steel is (50%~70%) scrap steel + (30%~50%) high-purity ferroalloy, wherein the scrap steel contains P≤0.015% and S≤0.010%, and the high-purity ferroalloy contains Fe-Mn≥98% and Fe-Si≥99%, using composite deoxidation (Al+Ce mixed deoxidation). Electromagnetic stirring (5Hz, 30kW) is applied for 10~15 minutes to ensure uniform composition, and the superheat of the molten steel is adjusted to 100~120℃.

[0011] Nano-coating; several cold cores are preheated to 245-255℃ and then fixed to the bottom of the mold, arranged in a regular array.

[0012] The thickness of the nano-coating is 50–300 μm.

[0013] The cold core has a tapered geometry: bottom diameter Φ50-80mm, top diameter Φ30-50mm, and the surface is polished to achieve a roughness of Ra6.3μm.

[0014] 3) Dynamic layered casting control: Casting is carried out in multiple layers, with Fe powder or Fe+Ce powder sprayed between each layer. Odd-numbered layers are sprayed with Fe powder, and even-numbered layers are sprayed with Fe+Ce powder. Infrared temperature measurement provides real-time feedback to adjust the casting interval (±2℃ accuracy). PIV monitoring controls the molten steel flow rate to 17.5~18.5kg / s, and the total casting time is controlled to 6.5~7.5 minutes.

[0015] The thickness of the sprayed powder layer is 1-4 mm; the casting time for each layer is 0.3-0.5 min, the casting interval is not less than 15 s, and the casting volume of each layer is 5%-25% of the total casting mass.

[0016] Temperature field: The cooling water flow rate of the crystallizer is dynamically adjusted to ΔT≤5℃; Flow field: End electromagnetic stirring, current 200-400A, frequency 3-8Hz.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] This invention achieves a breakthrough in improving the solidification quality of large ingots through the synergistic effect of cold-core composite structure design, dynamic layered casting control, and intelligent multi-physics field regulation. The specific innovative effects are as follows:

[0019] 1. Significantly increased proportion of equiaxed crystals: Through the directional nucleation effect of gradient cold core, the proportion of equiaxed crystals in the ingot is increased to over 65% (compared to only 40% in traditional processes), and the width of columnar crystal regions is compressed to ≤50mm, greatly improving the anisotropy of mechanical properties.

[0020] 2. Precise control of macrosegregation: The dynamic casting algorithm is adopted to reduce the macrosegregation index (GSI) to below 0.18% and narrow the fluctuation range of key elements (C, Mn) to ±0.05%, meeting the stringent standards for nuclear power forgings.

[0021] 3. Optimized shrinkage cavity distribution: The conical arrangement of the cold core makes the shrinkage cavity present a continuous conical distribution (volume percentage ≤2%), reducing the machining allowance by 40% and significantly improving material utilization.

[0022] 4. Significantly reduced risk of hot cracking: Real-time thermal stress compensation reduces the hot cracking susceptibility coefficient (HCS) from 0.35 to 0.22, resulting in a 75% reduction in scrap rate. Attached Figure Description

[0023] Figure 1 This is a plan view of the cold core distribution in Example 1.

[0024] Figure 2 This is a plan view of the cold core distribution in Example 2.

[0025] In the diagram: 1-Mold, 2-Cold core. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0027] At a pilot plant of a steel plant, three batches of rack steel were smelted using a 500kg vacuum induction furnace. The target composition of the experimental steel is shown in Table 1.

[0028] Table 1 Target Composition of Gear Steel (Mass Fraction) %

[0029]

[0030] Comparative Example 1:

[0031] Steelmaking method: A 500 kg vacuum induction furnace (model: VIM-500) is used, equipped with an electromagnetic stirring system (frequency: 5 Hz, power: 30 kW). Steel raw materials: Scrap steel (60%): AISI 4130 scrap, with strictly controlled P and S content (P≤0.015%, S≤0.010%). Ferroalloys (40%): High-purity Fe-Mn (Mn≥98%), Fe-Si (Si≥99%), Ni plates (Ni≥99.9%), Mo-Fe (Mo≥60%). Deoxidizer: Al blocks (Al≥99.5%, added at 0.05%). Vacuum degree: 6 Pa, melting temperature: 1585℃, steel superheat: 82℃. Electromagnetic stirring time: ≥15 min to ensure uniform composition.

[0032] Casting process: Sand casting (resin sand, cavity size Φ300 mm × 600 mm) was used, preheated to 120℃. Casting method: Single continuous casting, casting speed 18 kg / s, total casting time approximately 8 minutes. Cooling method: Natural cooling, no quenching measures were used.

[0033] Ingot quality analysis:

[0034] 1. Shrinkage cavities: Concentrated in the upper part of the ingot, with a depth of about 50 mm and a volume ratio of 8%.

[0035] 2. Segregation: C element fluctuation range 0.12%~0.28% (target 0.18%), Mn segregation ±15%.

[0036] 3. The proportion of columnar crystals is as high as 70%, while the equiaxed crystal region accounts for only 30%.

[0037] (Mainly), Class D, Grade 1.0 (spherical oxide).

[0038] Mechanical properties:

[0039] Tensile strength: 780 MPa (target ≥ 800 MPa), impact toughness: 45 J (target ≥ 60 J).

[0040] Example 1:

[0041] The superheat of the molten steel was increased to 110℃ to reduce the temperature gradient at the solidification front; Al+Ce mixed deoxidation was adopted, with Ce added at 0.02% (accounting for 0.02% of the total mass of the Al and Ce mixed deoxidizer) to reduce oxide inclusions. Other conditions for steelmaking were the same as in Comparative Example 1.

[0042] Casting process: The mold conditions are the same as those in Comparative Example 1.

[0043] Nano-coating; several cold cores are preheated and fixed to the bottom of the mold.

[0044] Dimensions: Bottom base Φ60 mm, top base Φ40 mm, height 80 mm, surface polished (Ra 6.3 μm). Preheat to 250℃. Avoid quenching of molten steel; fix to the bottom of the mold with refractory putty (see...). Figure 1 ), which are distributed in a cross-shaped symmetrical pattern (150 mm apart).

[0045] Casting method: layered casting, with the molten steel flow rate controlled at 18 kg / s; casting control parameters are shown in Table 2.

[0046] Table 2

[0047]

[0048] The Fe+Ce mixed powder consists of Fe powder (70%) and Ce (30%) with a particle size of 100-200 mesh. After spraying, it forms a liquid protective layer.

[0049] Temperature field: Crystallizer cooling water flow rate dynamically adjusted ΔT = 3℃; Flow field: End-effector electromagnetic stirring, current 250A, frequency 5Hz. Slow cooling for 48 hours.

[0050] Example 1: Ingot quality:

[0051] 1. The volume ratio of shrinkage cavities is reduced to 2%, concentrated in the top center (depth 20 mm), C segregation ±0.05%, Mn segregation ±0.05%.

[0052] 2. The proportion of equiaxed crystals has been increased to 65%, columnar crystals have been reduced, and inclusions are rated as: Class B, Grade 1.5, and Class D, Grade 0.5.

[0053] 3. Tensile strength: 810 MPa, impact toughness: 58 J.

[0054] Example 2:

[0055] The cold core arrangement was changed, and the number was increased to 6 (see...) Figure 2 The preheating temperature is 250℃ to enhance the nucleation effect. Other process measures are the same as in Example 1.

[0056] Dynamic temperature control: An infrared thermometer monitors the liquid surface temperature in real time and adjusts the casting interval accordingly.

[0057] The casting parameter adjustments for Example 2 are shown in Table 3.

[0058] Table 3:

[0059]

[0060] Ingot quality

[0061] Segregation control: C segregation degree ±0.05%, meeting the nuclear power forging standard.

[0062] Defect rate: shrinkage cavities account for only 2%, with no macroscopic porosity.

[0063] Mechanical properties: tensile strength 830 MPa, impact toughness 65 J.

[0064] The core advantages of layered casting: cold core promotes equiaxed crystal formation, segmented casting reduces segregation, and dynamic flux spraying effectively controls inclusions.

[0065] The comparison of inclusion quality between Example 2 and Comparative Example 1 is shown in Tables 4 and 5.

[0066] Table 4: Detailed List of Inclusion Classification, Rating, and Physical Properties

[0067] Mixed types ASTM E45 rating Main components (SEM-EDS) Morphological characteristics Uniformity of distribution Oxides (Type B) Level 1.0 (formerly Level 1.5) <![CDATA[ Al2O3-Ce2O3 composite ]]> Fine dispersion (1–5 μm) uniform Sulfides (Category A) Level 0.5 MnS + CeS Spherical / short rod-shaped No clustering Silicates (Category C) Not detected - - - Spherical oxides (Type D) Level 0.5 Ce-Al-O composite <3 μm random distribution

[0068] Table 5. Evaluation of Inclusions under Different Processes (Multi-dimensional Evaluation Table)

[0069] index Comparative Example 1 (Traditional Casting) Example 2 (Optimized Layered Casting) Improvement range Class B Oxide Rating Level 2.5 Level 1.0 ↓60% Type D spherical oxides Level 1.0 Level 0.5 ↓50% Inclusion density 15 pieces / mm² 5 pieces / mm² ↓67% Maximum inclusion size 20 μm 8 μm ↓60%

[0070] Composite inclusions (soft and fine) significantly improve hot working performance.

[0071] Compared with Comparative Example 1: Sulfide morphology control in Example 2: The addition of Ce changed MnS from a strip shape to a spherical shape, reducing the risk of banded structure during rolling.

[0072] The comparison shows that, compared with the traditional process (Comparative Example 1), the large rack steel ingots produced by the new layered casting process of the present invention (Examples 1 and 2) have significantly better internal quality, fewer inclusions, and can effectively solve the problems of macroscopic segregation and shrinkage.

[0073] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. For example, superheat and casting time are also limited to the essential scope of the present invention. Variations and modifications of the above embodiments will fall within the protection scope of the claims of the present invention.

Claims

1. A method for improving the solidification quality of large ingots, characterized in that, The method employing multi-point internal cold cores combined with composite layered casting includes the following steps: 1) Smelting and pretreatment: The superheat of the molten steel is 100-120℃; 2) Cold core system configuration: Cold cores are arranged in the mold. The cold core material adopts a three-layer gradient structure. The core is ZrO2-Y2O3 ceramic; the transition layer is Fe-Cr-Ni alloy; and the outer surface layer is plasma-sprayed Ce2O3 nano-coating. Several cold cores are preheated to 245-255℃ and then fixed to the bottom of the mold, and arranged in a regular array. 3) Dynamic layered casting control: casting is carried out in multiple layers, with Fe powder or Fe+Ce powder sprayed between each layer. Odd-numbered layers are sprayed with Fe powder, and even-numbered layers are sprayed with Fe+Ce powder. The steel flow rate is controlled at 17.5-18.5 kg / s, and the total casting time is controlled at 6.5-7.5 minutes. 4) Multi-field coupled solidification: Electromagnetic field: alternating frequency 1-5Hz, power density 3-10kW / m³ 3 ; Temperature field: The cooling water flow rate of the crystallizer is dynamically adjusted to ΔT≤5℃; Flow field: end electromagnetic stirring, current 200-400A, frequency 3-8Hz.

2. The method for improving the solidification quality of large ingots according to claim 1, characterized in that, The raw material ratio for molten steel is 50%–70% scrap steel + 30%–50% high-purity ferroalloy, the molten steel melting temperature is 1580–1600℃, and Al+Ce mixed deoxidation is used.

3. The method for improving the solidification quality of large ingots according to claim 1, characterized in that, In step 2), the thickness of the Ce2O3 nano-coating is 50-300 μm.

4. The method for improving the solidification quality of large ingots according to claim 1, characterized in that, The cold core has a tapered geometry: a bottom diameter of 50-80 mm and a top diameter of 30-50 mm.

5. The method for improving the solidification quality of large ingots according to claim 1, characterized in that, In the dynamic layered casting control in step 3), the thickness of the sprayed powder layer is 1-4 mm; the casting time for each layer is 0.3-0.5 min, the casting interval is not less than 15 s, and the casting amount of each layer is 5%-25% of the total casting mass.

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