Casting method and system for improving internal quality of 3-5t large ingot type cold work die steel

Abstract

The application belongs to the technical field of steel smelting, and discloses a casting method and system for improving the internal quality of 3-5t large ingot type cold work die steel. The casting method comprises the following steps: sequentially performing EAF furnace initial smelting, LF furnace refining, and VD vacuum degassing treatment to obtain molten steel with qualified components and up-to-standard cleanliness; placing a 3-5t steel ingot mold on an adjustable mechanical vibration platform, coating the inner wall of the steel ingot mold with special paint and drying, installing heat insulation plates at the riser position and preparing a heating agent; using a bottom casting process to cast the molten steel under the protection of inert gas, adding a heating and heat preservation covering agent on the molten steel surface after the molten steel fills the steel ingot mold, and then starting the adjustable mechanical vibration platform to apply mechanical vibration; demolding after the steel ingot is completely solidified, red delivery after heating, and rolling or forging to form a bar or a module. The application effectively solves the technical problems of coarse carbide and insufficient riser feeding of 3-5t large ingot type cold work die steel by combining the EAF+LF+VD refining route with the bottom casting and mechanical vibration processes.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, specifically relating to a casting method and system for improving the internal quality of 3-5 t large ingot cold work die steel. Background Technology

[0002] Cold work die steels, especially high-carbon, high-alloy ledeburitic steels (such as Cr12MoV, Cr12Mo1V1(D2), SKD11, etc.), are widely used in manufacturing dies for cold extrusion, cold blanking, and cold heading due to their high hardness, high wear resistance, and good hardenability. With the development of modern manufacturing towards large-scale and precision production, the demand for 3-5 ton ingot-sized cold work die steels is increasing. However, the production of large ingot-sized cold work die steels has long faced two major technical bottlenecks: firstly, the carbides are well-developed and unevenly distributed; secondly, the difficulty in riser feeding leads to severe internal porosity.

[0003] Firstly, regarding carbide control, the solidification characteristics of high-carbon, high-alloy cold work die steel dictate that a large amount of coarse ledeburite network bands and primary carbides will precipitate during solidification. For large ingots weighing 3-5 tons, due to the large cross-sectional dimensions, the solidification rate of the molten steel is extremely slow, resulting in a long duration of hot core formation. Carbides grow sufficiently at the grain boundaries, forming a continuous network or blocky distribution. This well-developed carbide structure has a strong heritability and is difficult to completely eliminate even during subsequent high-temperature diffusion annealing, rolling, and forging processes. After being rolled into bars, the carbide particles are usually coarse, with grain sizes often exceeding 10 μm, and exhibit significant agglomeration. The coarse and agglomerated carbides severely disrupt the matrix continuity and are highly likely to become crack initiation points during die service, leading to early die failure and significantly reducing the die's service life and reliability.

[0004] Secondly, in terms of riser feeding, large ingot casting generally faces the problem of low feeding efficiency. In traditional top-casting or static ingot casting, the solidification process of molten steel exhibits typical volume shrinkage characteristics. As the solidification front advances towards the ingot center, dendrites continuously grow and overlap, forming a closed dendritic network. This dendritic bridging phenomenon severely blocks the feeding channel between the riser and the ingot body, preventing the molten steel in the riser from effectively replenishing the solidification shrinkage in the center of the ingot body, thus forming defects such as central porosity and residual shrinkage cavities inside the ingot. This problem is particularly prominent for large ingots of 3 to 5 tons, often resulting in a riser removal rate as high as 15% to 20%, seriously reducing the yield and economy of the ingot.

[0005] In the existing technology, in order to solve the above problems, some companies adopt electroslag remelting process. Although it can improve the morphology and internal quality of carbides, it has the disadvantages of high energy consumption, long cycle and high cost, making it difficult to achieve large-scale industrial application.

[0006] For example, CN 115786816 B discloses a controllable eutectic carbide high-strength and high-toughness cold work die steel and its preparation method. This patent achieves controllable refinement of eutectic carbides through a combination of composition design and process control. The chemical composition is characterized by: C 0.6~0.7%, Cr 5.6~6.4%, Mo 1.5~2.0%, V 1.0~1.5%, Nb 0.6~0.9%, with the addition of rare earth elements La, Ce, and Y. The production process includes medium-frequency induction furnace smelting, secondary refining in a protective atmosphere electroslag furnace, forging or rolling, ultra-refining, annealing, and heat treatment or cryogenic treatment. The forging or rolling process can employ conventional methods, high-temperature diffusion annealing + forging or rolling, or secondary high-temperature diffusion annealing + secondary forging or rolling. Although the method in this patent involves the control of eutectic carbides in cold work die steel, it mainly refines the carbides through composition optimization (adding Nb and rare earth elements) and electroslag remelting + high-temperature diffusion annealing, which belongs to the composition design and high-temperature heat treatment path.

[0007] For example, CN 121344457 A discloses a large-slab cold work die steel and its preparation process. This die steel, by weight percentage, contains C: 0.69%–0.75%, Mn: ≤0.50%, Si: 0.70%–0.90%, S: ≤0.01%, P: ≤0.030%, Cr: 6.90%–7.10%, Mo: 1.90%–2.00%, V: 1.80%–2.00%, Al: ≤0.02%, with the balance being iron. Its preparation process includes: non-vacuum induction furnace melting, LF furnace refining, VD furnace vacuum treatment, casting electrode rods, electroslag remelting, multi-stage heating forging, and annealing. This application effectively solves the problems of severe carbide segregation and low internal crack detection pass rate in large-slab high-carbon die steel by optimizing the smelting and forging processes, ultimately obtaining large modules of 4.5t and above with uniform microstructure, high strength and toughness, and excellent wear resistance. Although the method in this application involves the control of eutectic carbides in large-slab cold work die steel, it mainly refines the carbides through electroslag remelting and multi-stage heating forging, which is costly.

[0008] For example, CN 118703869 A discloses a ledeburitic cold work die steel and its smelting method. The method includes sequentially smelting molten ledeburitic cold work die steel in an EBT electric arc furnace, refining it in an LF furnace, and treating it in a VD furnace, followed by low-temperature casting to obtain a steel billet. The raw material composition of the ledeburitic cold work die steel includes, by mass percentage: C 1.45%–1.7%, Si ≤0.40%, Mn ≤0.40%, S ≤0.001%, P ≤0.030%, Cr 11.5%–12.00%, Mo 0.45%–0.55%, and V 0.20%–0.30%, with the balance being iron. The steel billet is sequentially subjected to upsetting, forging, annealing, flaw detection, sampling, and finishing treatment to obtain the ledeburitic cold work die steel. This application addresses the issues of uniform and refined microstructure in cold work die steel from the perspective of steel microstructure control and key heat treatment technologies. Although the method in this application involves the homogenization control of the microstructure of cold work die steel, it mainly refines carbides through low-temperature casting and forging.

[0009] Other techniques have attempted to apply electromagnetic stirring during the ingot casting process, but for large ingots, electromagnetic stirring equipment requires significant investment, has limited penetration depth, and does not significantly improve the solidification structure of high-carbon steel. While bottom casting can reduce steel oxidation and improve the surface quality of ingots, bottom casting alone cannot fundamentally improve the solidification feeding conditions and carbide growth behavior inside large ingots.

[0010] Therefore, how to develop a casting method for 3-5 ton large ingot cold work die steel that can effectively refine carbides and improve their distribution morphology, and significantly improve riser feeding efficiency and reduce internal defects has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0011] This invention aims to overcome the shortcomings of existing technologies and provide a casting method and system for improving the internal quality of large ingot cold work die steel. By combining the EAF+LF+VD refining route with bottom casting and mechanical vibration processes, it solves the technical problems of coarse and uneven carbide distribution, poor riser feeding, and severe central porosity in 3-5t grade high carbon high alloy cold work die steel. It refines carbides, opens up feeding channels, reduces riser removal rate, improves yield and product performance, reduces production costs, and enhances market competitiveness.

[0012] To achieve the above objectives, the present invention adopts the following technical solution: According to a first aspect of the present invention, a casting method for improving the internal quality of 3-5 t grade large ingot cold work die steel is provided, comprising the following steps: S1 molten steel: successively undergoes primary refining in EAF furnace, refining in LF furnace, and VD vacuum degassing treatment to obtain molten steel with qualified composition and cleanliness. S2 bottom casting system preparation: Place the 3-5t steel ingot mold on an adjustable mechanical vibration platform, coat the inner wall of the steel ingot mold with a special coating and dry it, install the heat insulation plate at the riser and prepare the heating agent; S3 Casting and Mechanical Vibration Coupling Treatment: The molten steel is cast using a bottom casting process under inert gas protection. The casting speed of the ingot body is controlled at 80-150 kg / s, the casting speed of the riser is 5-30 kg / s, and the total casting time is 300-850 s. In the later stage of molten steel filling, a heating agent and a heat-insulating covering agent are added to the surface of the molten steel, and then the adjustable mechanical vibration platform is started to apply mechanical vibration. S4 Demolding and Subsequent Processing: After the steel ingot is completely solidified, it is demolded, heated in red, and then rolled or forged into bars or modules.

[0013] As a further implementation, in step S3, the vibration frequency of the mechanical vibration is 5 to 25 Hz, and the vibration amplitude is 0.2 to 5 mm.

[0014] As a further implementation, in step S3, the vibration time of the mechanical vibration is 10 to 30 minutes.

[0015] As a further implementation, in step S3, the mechanical vibration adopts a light vibration followed by a heavy vibration mode, where the vibration intensity in the light vibration stage is less than the vibration intensity in the heavy vibration stage.

[0016] As a further implementation, during the light vibration phase, the vibration frequency is 5–12 Hz and the vibration amplitude is 0.2–1 mm; during the heavy vibration phase, the vibration frequency is 13–25 Hz and the vibration amplitude is 1.5–5 mm.

[0017] As a further implementation, the duration of the light vibration phase accounts for 40-50% of the total vibration time.

[0018] As a further implementation method, in step S1, the EAF furnace primary refining uses scrap steel and pig iron as furnace charge, and controls the carbon content of the tapped steel to meet the lower limit of the corresponding steel grade through electric furnace melting, dephosphorization during oxidation period, removal of gas and inclusions, and adding alloys with the steel stream for pre-alloying during tapping, and adding composite deoxidizers for pre-deoxidation. The tapping temperature is 1660~1685℃.

[0019] As a further implementation, in step S1, the LF furnace refining control slag basicity CaO / SiO2 = 3.0~4.5, sulfur content ≤ 0.005%, white slag holding time ≥ 15 min, refining temperature ≥ 1560℃, and refining time ≤ 60 min.

[0020] As a further implementation, in step S1, the VD vacuum degassing treatment is maintained at a vacuum of ≤67Pa for ≥12min, with bottom blowing argon stirring, and soft blowing argon for ≥15min after vacuum breaking, and the superheat of the molten steel is controlled at 40~60℃.

[0021] As a further implementation, in step S1, the initial smelting in the EAF furnace is carried out in a 40t electric arc furnace.

[0022] As a further implementation, in step S3, the pouring temperature is controlled within the range of 1450 to 1470°C.

[0023] As a further embodiment, the cold work die steel is one of Cr12MoV, Cr12Mo1V1 or SKD11.

[0024] According to a second aspect of the present invention, a casting system for improving the internal quality of 3-5t grade large ingot cold work die steel is provided, which is used to implement the above casting method and includes: a 40t electric arc furnace, an LF refining furnace, a VD vacuum degassing furnace, a bottom casting device, and an adjustable mechanical vibration platform.

[0025] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art: 1. Carbide refinement and homogenization: Mechanical vibration breaks dendrites and promotes homogeneous nucleation, suppressing coarse network / block carbides. Carbide particles are ≤50μm and the thickness of the network ledeburite film is ≤25μm, significantly reducing crack initiation sites. 2. Improved riser feeding efficiency: Mechanical vibration eliminates dendrite bridging and opens up feeding channels, reducing riser shrinkage rate from 29.68% to 18.24% and riser removal rate to below 9%, thus reducing production costs; 3. Full-process collaborative quality control: EAF+LF+VD purifies molten steel, bottom casting reduces oxidation, mechanical vibration precisely controls solidification structure, and the entire process is adapted to the characteristics of 3-5t large ingot high carbon high alloy steel, solving exclusive technical pain points; 4. Simple process, low cost, and easy to industrialize: No electroslag remelting, electromagnetic stirring, or composition modification is required. Only a mechanical vibration platform is added to the conventional casting process. The equipment investment is low, the energy consumption is low, and the cycle is short, making it suitable for large-scale industrial production. 5. Significantly improved internal quality: central porosity ≤ 2.5 grade, flaw detection pass rate ≥ 96%, and mechanical properties (strength, toughness, wear resistance) are significantly better than conventional cast products.

[0026] This application achieves full-process synergistic optimization from purifying molten steel at the smelting source to controlling the structure during solidification by combining the EAF+LF+VD refining route with bottom casting and mechanical vibration processes. It effectively solves the technical problems of coarse carbides and insufficient riser feeding in 3-5t large ingot cold work die steel. Attached Figure Description

[0027] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the present invention will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0028] Figure 1 A flowchart of a casting method for improving the internal quality of 3-5 t grade large ingot cold work die steel provided by the present invention; Figure 2 Comparison of shrinkage cavity and ledeburite microstructure obtained by vibration casting and conventional casting. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0030] Specific embodiments of the invention are disclosed herein as needed; however, it should be understood that the embodiments disclosed herein are merely examples of the invention that may be implemented in various alternative forms. In the following description, various operating parameters and components are described in several contemplated embodiments. These specific parameters and components are provided as examples only and are not intended to be limiting.

[0031] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0032] The first aspect of this invention provides a casting method for improving the internal quality of 3-5 t large ingot cold work die steel, such as... Figure 1 As shown, the casting method includes the following steps: S1 molten steel: successively undergoes primary refining in EAF furnace, refining in LF furnace, and VD vacuum degassing treatment to obtain molten steel with qualified composition and cleanliness. S2 bottom casting system preparation: Place the 3-5t steel ingot mold on an adjustable mechanical vibration platform, coat the inner wall of the steel ingot mold with special coating and dry it, install insulation plates at the riser and prepare the heating agent; S3 Casting and Mechanical Vibration Coupling Process: The molten steel is cast using a bottom casting process and is poured under inert gas protection. In the later stage of molten steel filling, a heating agent and a heat-insulating covering agent are added to the surface of the molten steel, and then the adjustable mechanical vibration platform is started to apply mechanical vibration. S4 Demolding and Subsequent Processing: After the steel ingot is completely solidified, it is demolded, heated in red, and then rolled or forged into bars or modules.

[0033] The method of this invention combines the EAF+LF+VD refining route with bottom casting and mechanical vibration processes to achieve full-process synergistic optimization from purifying molten steel at the smelting source to controlling the structure during solidification. It effectively solves the technical problems of coarse carbides and insufficient riser feeding in 3-5t large ingot cold work die steel.

[0034] The principle of vibration casting is as follows: Under non-vibration conditions, due to the large temperature difference between the molten metal and the mold or crystallizer wall, the temperature gradient of the molten metal is large along the temperature drop direction, and its solidification form is intermediate solidification. When vibration is applied during the solidification process, the vibration can uniformly distribute the temperature of the molten metal, reduce the temperature gradient between different parts, widen the solidification area, and change the solidification mode of the molten metal from intermediate solidification to paste-like solidification, thereby expanding the range of the equiaxed crystal region.

[0035] When vibration is applied to molten metal, on the one hand, the excitation force forces the molten metal to undergo convective motion. The flowing melt washes away dendrites at the solid-liquid interface, causing periodic crystal shedding and proliferation at the interface, providing a nucleation substrate and particles for heterogeneous nucleation. Simultaneously, the forced flow of the melt homogenizes the temperature field of the molten metal, enhances its thermal conductivity, promotes homogeneous nucleation, and reduces alloy segregation. On the other hand, the vibration causes relative slippage between internal parts, leading to viscous shear due to velocity differences. Dendrites at the mold wall and floating dendrites in the molten metal are broken up and distributed within the molten pool, hindering the growth of columnar crystals at the mold wall. Furthermore, the vibration produces a cavitation effect on the molten metal, increasing the undercooling of nucleation, promoting nucleation, refining grains, promoting slag and gas precipitation and flotation, and enhancing molten metal feeding. It also affects the morphology, size, and distribution of the second phase in the alloy, thereby improving the microstructure of the casting and enhancing its overall performance.

[0036] Figure 2 The figures show a comparison of shrinkage cavities and ledeburite microstructures in the ingot body obtained by vibration casting and conventional casting, where Figure (a) corresponds to vibration casting and Figure (b) corresponds to conventional casting. Figure 2 It can be seen that: the steel ingots obtained by conventional casting have concentrated shrinkage cavities in the central area, and the central porosity is severe, with continuous or discontinuous voids. The ledeburite is distributed in a continuous network or coarse blocky form at the grain boundaries; while the steel ingots obtained by vibration casting have no concentrated shrinkage cavities, the central porosity is significantly reduced, and the network of ledeburite is significantly reduced.

[0037] The following provides further explanation of each step of the casting method.

[0038] In step S1, the EAF furnace (electric arc furnace) uses scrap steel and pig iron as furnace charge for primary refining. Through electric furnace melting, dephosphorization during oxidation, removal of gas and inclusions, the carbon content of the tapped steel is controlled to meet the lower limit of the corresponding steel grade. During the tapping process, alloys are added with the steel stream for pre-alloying, and composite deoxidizers are added for pre-deoxidation.

[0039] In some embodiments, the tapping temperature of the primary steelmaking process in the EAF furnace is 1660–1685°C.

[0040] In some embodiments, the primary smelting in the EAF furnace is carried out in a 40t electric arc furnace.

[0041] In step S1, after steel is tapped from the EAF furnace, the ladle is hoisted to the LF refining station for refining. The LF refining process includes slag formation and desulfurization, composition fine-tuning, and process control for the corresponding steel grade.

[0042] In some embodiments, the LF furnace refining control slag basicity CaO / SiO2 = 3.0 to 4.5, sulfur content ≤ 0.005%, and white slag retention time ≥ 15 min.

[0043] In some embodiments, the refining temperature of the LF furnace is ≥1560℃ and the refining time is ≤60min.

[0044] In step S1, after LF refining, the ladle is hoisted to the VD station for vacuum treatment. During the VD treatment process, the vacuum level is well controlled, the stirring is sufficient, and the soft blowing time is adequate to promote the removal of gases and inclusions by floating to the surface. The superheat of the molten steel is adjusted according to the casting requirements to provide suitable temperature conditions for subsequent casting.

[0045] In some embodiments, the VD vacuum degassing treatment is maintained at a vacuum of ≤67Pa for ≥12min, with bottom blowing argon stirring, followed by soft blowing argon for ≥15min after vacuum rupture, and the superheat of the molten steel is controlled at 40~60℃.

[0046] In step S2, a special mold coating is applied to the inner wall of the mold and allowed to dry fully. This protects the mold, improves the surface quality of the steel ingot, facilitates demolding, and reduces porosity defects. Insulation plates are installed at the riser of the steel ingot mold to reduce heat loss and enhance the riser's feeding capacity. An adjustable mechanical vibration platform is used, enabling stepless adjustment of frequency and amplitude.

[0047] In step S3, the molten steel, after passing the VD treatment, is smoothly poured from the bottom of the ingot mold through a casting car. The pouring process is protected by an inert gas (e.g., argon), and the filling of the molten steel should be smooth to avoid air entrapment and secondary oxidation. In the later stages of molten steel filling, a heating agent and an insulating covering agent are added to the surface of the molten steel to reduce surface heat loss, prevent premature crusting, prolong the riser liquid state time, enhance riser feeding, and reduce the riser removal rate. An insulation plate, combined with the insulating covering agent and heating agent, forms a complete riser insulation system to maximize the feeding effect. Specifically, the heating agent is added when the molten steel surface is 30-40mm from the top of the insulation plate; the insulating covering agent is added when the surface reaches the upper edge of the insulation. After a period of cooling, an adjustable mechanical vibration platform is activated to apply mechanical vibration. The insulating covering agent can be, for example, carbonized rice husk covering agent.

[0048] In some embodiments, during the casting process, the ingot pouring speed is controlled at 80–150 kg / s, the riser pouring speed at 5–30 kg / s, and the total pouring time at 300–850 s. Rapid ingot pouring ensures a short filling time, reduces oxidation during casting, provides higher-temperature molten steel, and increases the riser temperature, thus providing favorable conditions for vibration-assisted feeding. Slow riser pouring prolongs the liquid state time of the molten steel, strengthens riser feeding, and reduces central porosity. The total pouring time precisely matches the solidification characteristics of the large ingot, creating a uniform temperature field and laying the foundation for subsequent vibration-controlled microstructure.

[0049] In some embodiments, the vibration frequency of the mechanical vibration is 5–25 Hz, and the vibration amplitude is 0.2–5 mm. This vibration parameter range can both gently and uniformly create a temperature field in the molten steel and prevent surface cracking, while also effectively breaking up dendrites, refining carbides, and opening up feeding channels. The parameter range is adapted to the solidification characteristics of 3–5 t ingots, taking into account the microstructure control effect, equipment safety, and process stability.

[0050] In some embodiments, the vibration time of the mechanical vibration is 10 to 30 minutes. This vibration time range fully covers the key solidification stage of 3 to 5 t ingots, continuously breaking dendrites, refining carbides, and opening up feeding channels; it adapts to the uniform temperature field of the thermal inertia of the ingots, taking into account the effect of microstructure control, energy consumption and production efficiency.

[0051] In some embodiments, the mechanical vibration adopts a phased approach of light vibration followed by heavy vibration, with the vibration intensity in the light vibration phase being lower than that in the heavy vibration phase. This segmented mechanical vibration pattern, characterized by light vibration followed by heavy vibration, offers significant technical advantages compared to single light vibration or single heavy vibration: the initial light vibration applies gentle vibration at a lower frequency and smaller amplitude, enabling smooth vibration initiation, preventing molten steel from entrapping gas, splashing, and secondary oxidation, effectively reducing thermal stress on the ingot surface, and preventing surface cracking; simultaneously, it gently promotes molten steel convection, homogenizes the molten steel temperature field, induces heterogeneous nucleation in advance, and inhibits the directional growth of coarse columnar crystals, laying a good foundation for subsequent microstructure refinement. As the solidification process progresses, a high-frequency, high-amplitude vibration mode is switched to generate strong shearing and cavitation effects. This effectively breaks up the dendritic network at the solidification front, eliminates dendrite bridging, and opens up the feeding channel from the riser to the ingot core, significantly improving riser feeding efficiency and greatly reducing porosity and shrinkage defects in the ingot center. Simultaneously, the strong vibration continuously scours the solid-liquid interface, breaking down and refining ledeburite networks and coarse carbides, resulting in a fine, dispersed distribution of carbides and preventing the formation of continuous network or large blocky carbides, thus reducing crack initiation at its source. Furthermore, vibration promotes the removal of inclusions and bubbles from the molten steel, further purifying the steel and improving its uniformity and density. This invention, through a vibration sequence control that progresses from light to heavy, balances the stability of the ingot in the early stages of solidification, the microstructure control effect in the middle stages, and the safety of equipment operation. Without increasing costs or changing the existing main process, it significantly improves the internal quality and performance of 3-5 t large ingot cold work die steel, reduces riser removal rate, and increases yield.

[0052] In some embodiments, during the light vibration phase, the vibration frequency is 5–12 Hz and the vibration amplitude is 0.2–1 mm; during the re-vibration phase, the vibration frequency is 13–25 Hz and the vibration amplitude is 1.5–5 mm.

[0053] In some embodiments, the light vibration phase accounts for 40-50% of the total vibration time.

[0054] In some embodiments, in step S3, the pouring temperature is controlled within the range of 1450 to 1470°C.

[0055] The method of the present invention is applicable to high-carbon, high-alloy ledeburitic steels, including Cr12MoV, Cr12Mo1V1, or SKD11, etc.

[0056] A second aspect of this invention provides a casting system for improving the internal quality of 3-5 t grade large ingot cold work die steel. This system is used to implement the aforementioned casting method and includes: a 40 t electric arc furnace, an LF refining furnace, a VD vacuum degassing furnace, a bottom casting device, and an adjustable mechanical vibration platform. The 40 t electric arc furnace is used for primary steelmaking, the LF refining furnace for steel refining, the VD vacuum degassing furnace for vacuum degassing of the steel, the bottom casting device for casting operations, and the adjustable mechanical vibration platform for applying mechanical vibration.

[0057] The present invention will be further explained and described below with reference to specific embodiments.

[0058] Example 13.2t Flat D2 Cold Work Die Steel Ingot Production Case The method of this invention is used to produce 3.2t flat D2 cold work die steel ingots. The specific production steps include: Step 1: Primary smelting in a 40t electric arc furnace (1) Charging: Before charging, lime is placed at the bottom of the furnace, then the material is charged and energized for melting; (2) Melting: After the furnace charge is completely melted, adjust the temperature to 1600-1620℃ and stir with oxygen; (3) Oxidation: Oxygen blowing is used to decarbonize, degas, and remove inclusions at a temperature of 1630-1650℃; (4) Pre-reduction: Add appropriate amounts of lime, fluorite and deoxidizer, pre-reduction time is 15 min, and the whole process is supplemented by nitrogen blowing and stirring; (5) Adjust the temperature to 1660-1680℃ and tap the steel, and send it into the LF furnace for refining in a timely manner.

[0059] Step 2: LF furnace refining After tapping steel from the electric arc furnace, the ladle is hoisted to the LF refining station for refining. The LF refining process includes the following steps: (1) Slag formation and desulfurization: After 12-15 minutes of power refining, lime, fluorite and synthetic slag are added to form high basicity reducing slag. The basicity of the slag is controlled at CaO / SiO2=3.0-4.5 to fully desulfurize and control the sulfur content in the steel to ≤0.005%. The white slag retention time is ≥15 minutes. (2) Fine-tuning of composition: According to the composition requirements of the target steel grade, add alloys such as ferrochrome, ferromolybdenum, and ferrovanadium to adjust the content of alloying elements to the target range; (3) Temperature control: During the refining process, the temperature drop is compensated by electric arc heating, and the tapping temperature is ≥1560℃; (4) The refining time in the LF furnace should be controlled to be ≤45 min; (5) During the entire LF furnace refining process, strictly control the amount of carbon powder and steel slag, maintain a good reducing atmosphere, strengthen Ar blowing and stirring, quickly remove S, and strictly prohibit Al adjustment in the later stage of refining.

[0060] Step 3: VD vacuum degassing After LF refining, the ladle is hoisted to the VD station for vacuum treatment. The VD process parameters are as follows: (1) Vacuum degree: The holding time under a high vacuum degree of ≤67Pa is ≥12 minutes; (2) Argon blowing and stirring: Argon gas is blown in through the permeable bricks at the bottom of the ladle. The stirring intensity should be such that the molten steel is not exposed or splashed, so as to promote the removal of gas and inclusions by floating. (3) Break the air, add rare earth elements, measure the temperature, determine [H] or take [H] samples; (4) Soft blowing treatment: After the vacuum is broken, soft blowing argon operation is performed. The soft blowing time is ≥15 minutes to allow the inclusions in the molten steel to float up fully and purify the molten steel. (5) Pouring temperature of the ladle: T=1450~1460℃.

[0061] Step 4: Preparation of the bottom casting system The bottom casting method is adopted. A 3.2t flat steel ingot mold is placed on a vibrating platform, and an insulation plate is installed. The slag is suspended 300mm from the bottom. The heating agent is prepared, and the casting is carried out in two batches. The vibrating platform adopts an adjustable mechanical vibration device, which can realize stepless adjustment of frequency and amplitude.

[0062] Step 5: Coupling treatment of casting and mechanical vibration After the VD treatment is qualified, the molten steel is smoothly poured from the bottom of the ingot mold through the casting car. Argon gas is used for protection during the casting process. The casting speed of the ingot body is 80-100 kg / s, the casting speed of the riser is 5-20 kg / s, and the total casting time is 300-320s. The filling of the molten steel mold should be stable to avoid air entrapment and secondary oxidation.

[0063] After the molten steel is filled, a heat-generating and heat-insulating covering agent is added. Then, the vibration platform is started to apply mechanical vibration to the steel ingot mold, starting with light vibration and gradually increasing the intensity. The vibration parameters are as follows: (1) Vibration frequency: 10-15 Hz; (2) Vibration amplitude: 0.5~1mm; (3) Vibration direction: horizontal circular vibration; (4) Vibration time: 12 min, with light vibration for the first 5 min and heavy vibration for the last 7 min.

[0064] Step Six: Demolding and Subsequent Processing The steel ingot is cooled for 120 minutes and then demolded to obtain a 3.2t flat D2 cold work die casting steel ingot, which is then sent to the next process for hot deformation to obtain the finished product.

[0065] Testing revealed that the maximum carbide particle size of the finished product was ≤35μm, the ledeburite film thickness was ≤18μm, vibration casting resulted in a riser shrinkage rate of 6.28% and an ingot shrinkage rate of 3.2%, which refined the eutectic ledeburite network. The carbide rating was ≤1 grade; the central porosity was ≤0.5 grade, and the ultrasonic flaw detection pass rate was ≥98%.

[0066] Comparative Example 1 This comparative example uses the exact same smelting, refining, bottom casting preparation, and pouring parameters as Example 1, except that mechanical vibration is not applied. All other process parameters are the same, and it is used to compare and verify the technical effect of mechanical vibration.

[0067] The finished product was tested and found to have a maximum carbide particle size of ≤64μm, a ledeburite film thickness of ≤23μm, a riser shrinkage rate of 29.68%, a spindle shrinkage rate of 6.7%, a carbide rating of ≤1.5, a central porosity of ≤1, and an ultrasonic flaw detection pass rate of ≥96%.

[0068] Example 25: Production of Rectangular Cr12MoV Cold Work Die Steel Ingot The method of this invention is used to produce 5t rectangular Cr12MoV cold work die steel ingots. The specific production steps include: Step 1: Primary smelting in a 40t electric arc furnace (1) Charging: Before charging, lime is placed at the bottom of the furnace, then the material is charged and energized for melting; (2) Melting: After the furnace charge is completely melted, adjust the temperature to 1600-1620℃ and stir with oxygen; (3) Oxidation: Oxygen blowing is used to decarbonize, degas, and remove inclusions at a temperature of 1625-1655℃; (4) Pre-reduction: Add appropriate amounts of lime, fluorite and deoxidizer. The pre-reduction time is 20 min. The whole process is supplemented by nitrogen blowing and stirring. (5) Adjust the temperature to 1675-1685℃ and tap the steel, and send it into the LF furnace for refining in time.

[0069] Step 2: LF furnace refining After tapping steel from the electric arc furnace, the ladle is hoisted to the LF refining station for refining. The LF refining process includes the following steps: (1) Slag formation and desulfurization: After 15-20 minutes of power refining, lime, fluorite and synthetic slag are added to form high basicity reducing slag. The slag alkali is controlled to CaO / SiO2=3.0-4.5 to fully desulfurize and control the sulfur content in the steel to ≤0.005%. The white slag retention time is ≥18 minutes. (2) Fine-tuning of composition: According to the composition requirements of the target steel grade, add alloys such as ferrochrome, ferromolybdenum, and ferrovanadium to adjust the content of alloying elements to the target range; (3) Temperature control: During the refining process, the temperature drop is compensated by electric arc heating, and the tapping temperature is ≥1565℃; (4) The refining time in the LF furnace should be controlled to be ≤60 min; (5) During the entire LF furnace refining process, strictly control the amount of carbon powder and steel slag, maintain a good reducing atmosphere, strengthen Ar blowing and stirring, quickly remove S, and strictly prohibit Al adjustment in the later stage of refining.

[0070] Step 3: VD vacuum degassing After LF refining, the ladle is hoisted to the VD station for vacuum treatment. The VD process parameters are as follows: (1) Vacuum degree: The holding time under a high vacuum degree of ≤67Pa is ≥15 minutes; (2) Argon blowing and stirring: Argon gas is blown in through the permeable bricks at the bottom of the ladle. The stirring intensity should be such that the molten steel is not exposed or splashed, so as to promote the removal of gas and inclusions by floating. (3) Break the air, add rare earth elements, measure the temperature, determine [H] or take [H] samples; (4) Soft blowing treatment: After the vacuum is broken, soft blowing argon operation is performed. The soft blowing time is ≥20 minutes to allow the inclusions in the molten steel to float up fully and purify the molten steel. (5) Pouring temperature of the ladle: T=1460~1470℃.

[0071] Step 4: Preparation of the bottom casting system The bottom casting method is adopted. A 5t rectangular steel ingot mold is placed on a vibrating platform, and an insulation plate is installed. The slag is suspended 300mm from the bottom. The heating agent is prepared, and the ingot is poured in one batch. The vibrating platform adopts an adjustable mechanical vibration device, which can realize stepless adjustment of frequency and amplitude.

[0072] Step 5: Coupling treatment of casting and mechanical vibration After VD treatment is qualified, molten steel is smoothly injected from the bottom of the ingot mold through a casting car. Argon gas is used for protection during the casting process. The casting speed of the ingot body is 120-140 kg / s, the casting speed of the riser is 10-25 kg / s, and the total casting time is 760-800s. The filling of the molten steel mold should be stable to avoid air entrapment and secondary oxidation.

[0073] After the molten steel is filled, a heat-generating and heat-insulating covering agent is added. Then, the vibration platform is started to apply mechanical vibration to the steel ingot mold, starting with light vibration and gradually increasing the intensity. The vibration parameters are as follows: (1) Vibration frequency: 15-20Hz; (2) Vibration amplitude: 2-3 mm; (3) Vibration direction: horizontal circular vibration; (4) Vibration time: 20 min, with light vibration for the first 8 min and heavy vibration for the last 12 min.

[0074] Step Six: Demolding and Subsequent Processing The steel ingot is cooled for 180 minutes and then demolded to obtain a 5t rectangular Cr12MoV cold work die casting steel ingot, which is then sent to the next process for hot deformation to obtain the finished product.

[0075] The finished product was tested and found to have a maximum carbide particle size of ≤50μm, a ledeburite film thickness of ≤25μm, a riser shrinkage rate of 18.24%, a spindle shrinkage rate of 9.74%, a carbide rating of ≤2.5, a central porosity of ≤2.5, and an ultrasonic flaw detection pass rate of ≥96%.

[0076] Comparative Example 2 This comparative example uses the exact same smelting, refining, bottom casting preparation, and pouring parameters as Example 2, except that mechanical vibration is not applied. All other process parameters are the same, and it is used to compare and verify the technical effect of mechanical vibration.

[0077] The finished product was tested and found to have a maximum carbide particle size of ≤83μm, a ledeburite film thickness of ≤37μm, a riser shrinkage rate of 29.68% and an ingot shrinkage rate of 13.21% due to vibration casting, a carbide rating of ≤3.5, a central porosity of ≤3.5, and an ultrasonic flaw detection pass rate of ≥92%.

[0078] As can be seen from the above embodiments and corresponding comparative examples, compared with conventional casting, the vibration casting of the present invention significantly refines carbides, eliminates network structure, greatly reduces central porosity, and improves the pass rate of flaw detection and the yield rate, playing a key role in improving the quality of 3-5t grade large ingot cold work die steel.

[0079] In summary, this invention, through the EAF+LF+VD refining route combined with bottom casting and mechanical vibration processes, achieves full-process synergistic optimization from purifying molten steel at the smelting source to controlling the microstructure during solidification, effectively solving the technical problems of coarse carbides and insufficient riser feeding in 3-5 t large ingot cold work die steel.

[0080] Finally, it should be noted that the embodiments described above are only some, not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A casting method for improving the internal quality of 3-5 t large ingot cold work die steel, characterized in that, Includes the following steps: S1 molten steel: successively undergoes primary refining in EAF furnace, refining in LF furnace, and VD vacuum degassing treatment to obtain molten steel with qualified composition and cleanliness. S2 bottom casting system preparation: Place the 3-5t steel ingot mold on an adjustable mechanical vibration platform, coat the inner wall of the steel ingot mold with a special coating and dry it, install the heat insulation plate at the riser and prepare the heating agent; S3 Casting and Mechanical Vibration Coupling Treatment: The molten steel is cast using a bottom casting process under inert gas protection. The casting speed of the ingot body is controlled at 80-150 kg / s, the casting speed of the riser is 5-30 kg / s, and the total casting time is 300-850 s. In the later stage of molten steel filling, a heating agent and a heat-insulating covering agent are added to the surface of the molten steel, and then the adjustable mechanical vibration platform is started to apply mechanical vibration. S4 Demolding and Subsequent Processing: After the steel ingot is completely solidified, it is demolded, heated in red, and then rolled or forged into bars or modules.

2. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 1, characterized in that, In step S3, the vibration frequency of the mechanical vibration is 5 to 25 Hz and the vibration amplitude is 0.2 to 5 mm.

3. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 1, characterized in that, In step S3, the vibration time of the mechanical vibration is 10 to 30 minutes.

4. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 2, characterized in that, In step S3, the mechanical vibration adopts a light vibration followed by a heavy vibration mode, and the vibration intensity in the light vibration stage is less than that in the heavy vibration stage.

5. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 4, characterized in that, During the light vibration phase, the vibration frequency is 5–12 Hz and the vibration amplitude is 0.2–1 mm; during the heavy vibration phase, the vibration frequency is 13–25 Hz and the vibration amplitude is 1.5–5 mm; the duration of the light vibration phase accounts for 40–50% of the total vibration time.

6. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 1, characterized in that, In step S1, the EAF furnace primary refining uses scrap steel and pig iron as furnace charge. Through electric furnace melting, dephosphorization during oxidation, and removal of gases and inclusions, the carbon content of the tapped steel is controlled to meet the lower limit of the corresponding steel grade. During the tapping process, alloys are added with the steel stream for pre-alloying, and composite deoxidizers are added for pre-deoxidation. The tapping temperature is 1660-1685℃. The LF furnace refining controls the slag basicity CaO / SiO2 = 3.0-4.5, sulfur content ≤0.005%, white slag holding time ≥15min, refining temperature ≥1560℃, and refining time ≤60min. The VD vacuum degassing treatment is maintained at a vacuum of ≤67Pa for ≥12min, with bottom blowing argon stirring. After vacuum breaking, soft blowing argon is carried out for ≥15min, and the superheat of the molten steel is controlled at 40-60℃.

7. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 1, characterized in that, In step S1, the initial smelting in the EAF furnace is carried out in a 40t electric arc furnace.

8. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 1, characterized in that, In step S3, the pouring temperature is controlled within the range of 1450 to 1470°C.

9. The casting method for improving the internal quality of 3-5t grade large ingot cold work die steel according to claim 1, characterized in that, The cold work die steel is one of Cr12MoV, Cr12Mo1V1 or SKD11.

10. A casting system for improving the internal quality of 3-5 t large ingot cold work die steel, characterized in that, The casting method for implementing any one of claims 1-9 includes: a 40t electric arc furnace, an LF refining furnace, a VD vacuum degassing furnace, a bottom casting device, and an adjustable mechanical vibration platform.

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