Improved die steel with high mirror polishing characteristic and preparation method

Abstract

The invention relates to improved die steel with a high mirror polishing characteristic and a preparation method. The material is composed of the following raw materials in percentage by mass: 0.30-0.42% of C; si: 0.71 to 1.20%; 0.12% to 0.53% of Mn; cr: 4.50 to 5.50%; 2.01% to 3.02% of Mo; 0.30% to 0.61% of V; p: less than or equal to 0.012%; s: < = 0.002%; 0.01%-0.10% of synergistic elements; and the balance of Fe and other inevitable impurities. And the synergistic elements comprise 0.008-0.012% of F, 0.03-0.035% of rare earth, 0.03-0.04% of Nb and 0.002-0.003% of B. According to the invention, the synergistic elements (F, rare earth, Nb and B form a synergistic system) are added, so that the high-mirror-surface polishing performance, the anti-pollution performance, the durability and the like are achieved.

Description

Technical Field

[0001] This application relates to the field of metallic materials technology, and more specifically, to an improved mold steel with high mirror polishing properties and a method for its preparation. Background Technology

[0002] In the field of metal materials technology, mold steel, as a key basic material for mold manufacturing, plays a crucial role in the quality and production efficiency of mold manufacturing and related industrial products. With the rapid development of science and technology and the continuous upgrading of industrial production, various industries are placing increasingly higher demands on the precision, surface quality, and service life of molds. In high-end manufacturing industries such as automobiles, electronics, and aerospace, the mirror polishability and corrosion resistance of molds directly affect the appearance, optical performance, and environmental resistance of products. For example, in automotive headlight manufacturing, high-mirror-finish molds ensure that injection-molded transparent plastic lenses or metal reflectors possess excellent optical performance, improving the lighting effect and aesthetics of automobiles. In electronic device housing manufacturing, good corrosion resistance ensures that molds can be used for a long time in harsh environments such as humidity and high temperatures without corrosion, thus guaranteeing the dimensional accuracy and surface quality of the products. Therefore, improving the mirror polishability and corrosion resistance of mold steel has become an important research topic in the field of metal materials, and the continuous development and innovation of related technologies are constantly driving the mold manufacturing industry towards higher precision, higher efficiency, and higher reliability.

[0003] To improve the mirror polishability and corrosion resistance of mold steel, existing technologies typically employ adjustments to the alloy composition. A common approach is to add a high content of chromium, combined with appropriate amounts of carbon, nickel, and other elements, to enhance the material's polishing effect and corrosion resistance. Chromium forms a dense passivation film on the surface of mold steel, effectively preventing external media from eroding the material and thus improving its corrosion resistance. The addition of carbon, nickel, and other elements improves the material's microstructure, resulting in finer and more uniform grains, thereby enhancing polishing performance. Another research direction involves introducing trace elements such as titanium, molybdenum, and vanadium to further optimize the mirror finish and toughness of mold steel. These trace elements can refine grains, improve the material's strength and toughness, and also contribute to improved polishing performance. Titanium forms stable carbides, pinning grain boundaries and preventing grain growth; molybdenum increases the material's high-temperature strength and hardness; and vanadium helps improve the material's wear resistance and fatigue resistance. Furthermore, some studies attempt to further optimize the performance of mold steel by adjusting the proportions and order of addition of alloying elements.

[0004] However, these traditional solutions have significant drawbacks in long-term practical use. Firstly, mold steel easily absorbs contaminants during processing and use, making it difficult to clean and severely impacting the durability of the mirror finish. During mold manufacturing, processes such as electrical discharge machining, wire cutting, and precision grinding cause localized high-temperature oxidation and static electricity, leading to the adhesion of metal shavings, dust, and other fine particles on the surface, making cleaning cumbersome. Furthermore, in subsequent injection molding, volatile organic compounds, siloxane residues, and vapor-deposited particles from high-temperature molten plastic, silicone sealing materials, and metal plating processes easily carbonize or adhere to the cavity surface, forming stubborn stains. Because the mirror surface is extremely smooth and cannot tolerate mechanical scratches, frequent cleaning carries the risk of surface damage or secondary contamination, making it difficult to maintain the mirror finish quality of the mold over long-term use. Secondly, molds must withstand alternating cycles of heating and cooling during long-term use, which can easily lead to dimensional deformation, severely affecting the mold's forming accuracy and shortening its lifespan. Summary of the Invention

[0005] The purpose of this application is to further improve the anti-fouling performance and durability (achieving higher anti-fouling retention rate and dimensional retention rate after cold and hot aging tests) on the basis of meeting the basic performance of molds (high mirror polishing characteristics, corrosion resistance, impact toughness, and low coefficient of thermal expansion), and to provide an improved mold steel with high mirror polishing characteristics and its preparation method.

[0006] Firstly, an improved mold steel with high mirror polishing properties is composed of the following raw materials by mass fraction: C: 0.30-0.42%; Si: 0.71-1.20%; Mn: 0.12-0.53%; Cr: 4.50-5.50%; Mo: 2.01-3.02%; V: 0.30-0.61%; P: ≤0.012%; S: ≤0.002%; Synergistic element: 0.01-0.10% The balance consists of Fe and other unavoidable impurities; The enhancing elements include: F 0.008-0.012%, rare earth 0.03-0.035%, Nb 0.03-0.04%, and B 0.002-0.003%.

[0007] This application utilizes a quaternary microalloying enhancement system of F, rare earth, Nb, and B, combined with a specific mass fraction of C, Si, Mn, Cr, Mo, V, P, S, enhancement elements, and a balance of Fe and other unavoidable impurities in the improved mold steel. By leveraging the directional aggregation guiding effect of rare earth elements, precise functional zoning control of F (surface layer), B (grain boundary), and Nb (matrix) is achieved. While ensuring that the core basic performance of the mold steel (high mirror polishability, corrosion resistance, impact toughness, and low coefficient of thermal expansion) fully meets the standards, this application breaks through the technical pain points of traditional high mirror mold steel, namely "poor anti-fouling stability and easy deformation under thermal cycling," achieving a synergistic leap in anti-fouling performance and durability, especially demonstrating excellent performance retention under long-term alternating hot and cold service scenarios.

[0008] The mold steel obtained by this application has a thermal expansion coefficient ≤12.2*10-6m / m•K, thermal conductivity ≥30W / (m・K), fine and coarse inclusions of Class A, B, and C are all ≤0.5 grade; fine and coarse inclusions of Class D (spherical oxides) are all ≤1.0 grade, impact toughness ≥120J / cm², salt spray test time ≥120h, pollutant contact angle ≥130°, dust adhesion rate ≤2%, and after a cold and hot cycle aging test, the cycle conditions are -40℃×2h→room temperature 25℃×1h→400℃×2h, with a single cycle period of 5h, and after a cumulative cycle of 100 times, the dust adhesion rate retention rate and dimensional retention rate are both above 95%.

[0009] Preferably, the rare earth element is a mixed rare earth element, which includes Ce and La.

[0010] By setting rare earth elements as a mixed rare earth element containing Ce and La, and combining it with F, Nb, and B to form a quaternary microalloying enhancement system, the dimensional stability, anti-fouling stability, and anti-fouling performance of molds can be further improved after long-term use, based on high mirror polishing characteristics, low coefficient of thermal expansion, and corrosion resistance. Through the directional segregation guiding effect of rare earth elements, precise functional zoning control of F, B, and Nb can be achieved, synergistically realizing grain boundary purification, matrix densification, grain boundary strengthening, and low surface energy forming of the surface layer. It can eliminate harmful P / S impurities and non-metallic inclusions in steel, inhibit P / S grain boundary segregation, eliminate brittle grain boundary phases, modify irregular inclusions into spherical low surface energy inclusions, eliminate polishing defect sources and dust adsorption sites, and open up channels for the directional segregation of F / B. It can eliminate micro-stress concentration in the matrix, so that the matrix structure is free from coarsening and segregation during forging / heat treatment, forming an inherently stable microstructure, laying the foundation for strength, toughness, and resistance to thermal fatigue, while ensuring the stability of the low coefficient of thermal expansion.

[0011] Preferably, the mass fraction ratio of Ce to La is 7:3.

[0012] In improved mold steels composed of specific mass fractions of C, Si, Mn, Cr, Mo, V, P, S, synergistic elements, and the balance Fe and other unavoidable impurities, when the synergistic elements contain a specific proportion of F, rare earth, Nb, and B, and the rare earth is a mixed rare earth containing Ce and La, with a Ce to La mass fraction ratio of 7:3, the dual purification effect of rare earth on harmful P / S impurities and non-metallic inclusions in the steel can be further optimized. This better suppresses P / S grain boundary segregation, eliminates brittle grain boundary phases, modifies irregular inclusions into spherical low surface energy inclusions, eliminates polishing defect sources and dust adsorption sites, and opens up better channels for the directional segregation of F / B. This enhances the synergistic system effect of "rare earth as the core link, Nb as the microstructure matrix, F as the surface properties control, and B as the grain boundary mechanics," further improving the ultra-mirror polishing performance, long-term anti-fouling performance, toughness, and thermal fatigue resistance of the mold steel, and ensuring dimensional stability and low thermal expansion coefficient after heat treatment.

[0013] Secondly, a method for preparing an improved mold steel with high mirror polishing properties includes the following steps: 1) After mixing all raw materials except for the synergistic elements evenly, proceed with roughing and refining in sequence to form mixture A; 2) Addition of synergistic elements: According to the mass fraction of the formula, add rare earth fluoride composite agent, mix evenly, then add nano boron iron alloy powder, mix evenly, then add niobium iron powder, mix evenly, to obtain mixture B; 3) Then, the process of casting, forging, quenching, tempering, stress relief, and polishing is carried out in sequence to obtain the improved mold steel.

[0014] First, the raw materials, excluding the enhancing elements, are mixed and then coarsely and refined to initially remove impurities and fully integrate the raw materials. Then, rare earth fluoride composites, nano-boron-iron alloy powder, and niobium-iron powder are added according to the formula, allowing the enhancing elements F, rare earth, Nb, and B to be evenly distributed in the mold steel in proportion. Utilizing the synergistic effect of the quaternary microalloying enhancing system, grain boundary purification, matrix densification, grain boundary strengthening, and low surface energy forming are achieved. Subsequent processes, including casting, forging, quenching, tempering, stress relief, and polishing, enable the mold steel to form a stable microstructure, improving its strength, toughness, thermal fatigue resistance, high-temperature tempering stability, and thermal cycling stability. This results in an improved mold steel with high mirror polishing properties, long-term stain resistance, and dimensional stability.

[0015] Preferably, the quenching process is carried out at a temperature of 1020-1050℃, and the cooling method is vacuum quenching or oil quenching.

[0016] When preparing improved mold steel with high mirror polishing properties, controlling the quenching process temperature at 1020-1050℃ and using vacuum quenching or oil quenching cooling methods can ensure that the mold steel obtains suitable microstructure and properties during the quenching process. Combined with the fact that the improved mold steel is composed of raw materials with a specific mass fraction, including synergistic elements such as F, rare earth, Nb, and B, it can further ensure that the mold steel achieves properties such as ultra-mirror polishing, long-term anti-fouling, improved strength and toughness, and improved thermal fatigue resistance, maintaining high surface quality and precise dimensional accuracy of the mold during long-term use.

[0017] Preferably, the temperature of the tempering process is ≤500℃.

[0018] When preparing improved mold steel with high mirror polishing properties, the tempering process temperature is controlled at ≤500℃. Combined with the fact that mold steel is composed of specific mass fractions of C, Si, Mn, Cr, Mo, V, P, S, synergistic elements (including F, rare earth, Nb, B), Fe, and unavoidable impurities, and the preparation method of first mixing raw materials except for synergistic elements for roughing and refining, then adding synergistic elements, and finally performing casting, forging, quenching, tempering, stress relief, and polishing in sequence, this method helps to improve the performance of mold steel, ensure the dimensional stability, anti-fouling stability, and anti-fouling performance of mold steel after long-term use (alternating hot and cold), and achieve all-dimensional performance enhancement such as ultra-mirror polishing, long-term anti-fouling, improved strength and toughness, and improved resistance to thermal fatigue.

[0019] Preferably, the stress-relieving temperature is <300℃, and the cooling method is: furnace cooling ≤500℃ air cooling.

[0020] Improved mold steel with high mirror polishing properties is composed of C, Si, Mn, Cr, Mo, V, P, S, and synergistic elements in a specific mass fraction. The synergistic elements include F, rare earth elements, Nb, and B. The rare earth elements are a mixture of Ce and La with a Ce / La mass fraction ratio of 7:3. The process involves first mixing and refining the raw materials (excluding the synergistic elements), then adding rare earth fluoride composites, nano-boron-iron alloy powder, and niobium-iron powder in sequence. Finally, the steel is produced through casting, forging, quenching, tempering, stress relief, and polishing. The stress relief temperature is <300℃, and the cooling method is furnace cooling to air cooling at ≤500℃. This process enables the mold steel to achieve ultra-mirror polishing, long-term stain resistance, improved strength and toughness, and enhanced resistance to thermal fatigue. It ensures dimensional stability and a low coefficient of thermal expansion after heat treatment. At the same time, the stress relief treatment eliminates internal stress in the mold steel, preventing deformation and cracking caused by stress concentration, further improving the dimensional stability and service life of the mold steel.

[0021] In summary, this application includes at least one of the following beneficial technical effects: 1. F, rare earth, Nb, and B synergistically construct a synergistic system with "rare earth as the core link, Nb determining the microstructure, F controlling surface properties, and B strengthening grain boundary mechanics", achieving a dual performance closed loop of grain boundary purification and densification, ultra-mirror polishing and long-term anti-fouling, grain boundary strengthening and microstructure stabilization, and improved toughness and thermal fatigue resistance. This effectively solves the problem that traditional mold steels are difficult to balance in terms of polishability and anti-fouling properties, and high hardness and toughness. 2. During heat treatment, the synergistic elements and Cr / Mo generate CrF3 / MoF6 composite low surface energy compounds, which cover the surface of the passivation film, reduce the surface energy of the mirror, significantly weaken the adsorption force between dust and the mirror, and at the same time, the fluoride layer has high hardness and scratch resistance, achieving a long-term anti-fouling effect. 3. The synergistic effect of Nb, B, and F enhances the high-temperature tempering stability and thermal cycling stability of mold steel, preventing grain boundary micro-deformation, matrix volume abrupt change, and surface oxidation damage during repeated thermal cycling, ensuring the integrity of the mold mirror surface and extending the mold service life. Detailed Implementation

[0022] The present application will be further described in detail below with reference to the embodiments. Example

[0023] The preparation processes for Examples 1-3 are as follows: An improved die steel with high mirror polishing properties is prepared by the following method: 1. Mix all raw materials except for the synergistic elements (C, Si, Mn, Cr, Mo, V, P, S, some Fe and other unavoidable impurities) evenly, and then perform roughing and refining in sequence to produce mixture A; Rough smelting: The uniformly mixed raw materials are put into an electric arc furnace or induction furnace, heated to 1550℃, with a heating rate of 5 min / 30℃, and held at the temperature for 2 hours to obtain molten steel.

[0024] Refining: The molten steel after roughing is transferred into a refining furnace (VOD furnace), heated to 1720℃, held for 2 hours, with a heating rate of 3 min / 30℃, and the initial vacuum degree is controlled at 133 Pa. As the holding time progresses, the vacuum degree is reduced to 13 Pa at 1 Pa / min to obtain mixture A.

[0025] 2) Addition of enhancing elements: Cerium fluoride and lanthanum fluoride are injected into the molten steel through a hopper using nitrogen as a carrier gas, with a carrier gas pressure of 0.4 MPa and a blowing time of 8 min. During the blowing process, the bottom blowing argon flow rate is maintained at 150 NL / min. After the blowing is completed, the mixture is allowed to stand for 5 min. Addition of nano-boron-iron alloy powder: Nano-boron-iron alloy powder (average particle size 100 nm, boron content 20 wt%) is loaded into a sealed container and added to the molten steel through a vacuum feed tube at a rate of 2 kg / min. During the addition process, the argon flow rate is increased to 250 NL / min for strong stirring. After the addition is completed, strong stirring is maintained for 5 min, followed by weak stirring (80 NL / min) for 10 min. Then, niobium iron powder (niobium content 65 wt%, average particle size 0.001 mm) is added to the molten steel at a rate of 3 kg / min. After the addition is completed, the temperature is raised to 1620℃ and held for 15 min to ensure that the elements are fully dissolved and homogeneous, resulting in mixture B. 3) Then proceed with the following steps in sequence: casting, forging, quenching, tempering, stress relief, and polishing, as detailed below: Casting: The mixture B is lifted out of the vacuum tank and injected into a cast iron ingot mold with an inner cavity size of Φ300mm×600mm under argon protection. The casting temperature is controlled at 1550℃ and the casting time is controlled at 45s to obtain a steel ingot. Forging: The steel ingot is heated to 1180℃ at a heating rate of 100℃ / h and held at that temperature for 3 hours before being taken out of the furnace for forging. The initial forging temperature is 1100℃ and the final forging temperature is 900℃. A three-dimensional forging method is adopted: first, axial elongation; then, radial upsetting; and finally, axial elongation to the predetermined size. The total forging ratio is 6. After forging, the ingot is air-cooled to room temperature (25℃). Quenching: Heat the forging to 1020℃ at a heating rate of 80℃ / h and a holding coefficient of 1.2min / mm (calculated based on the effective thickness). After holding, remove the forging from the furnace and cool it in oil at a temperature of 80℃. Cool it to below 150℃ and then remove it from the oil and air cool it. Tempering: Heat the quenched workpiece to 500℃ at a heating rate of 100℃ / h, hold for 2.5h, and then air cool after removing it from the furnace; repeat this tempering process three times (i.e., three temperings), with the same tempering temperature each time, and air cool to room temperature after each tempering. Stress relief: Heat the tempered workpiece to 250℃, hold for 4 hours, and cool it by furnace cooling at 100℃ followed by air cooling, and then remove it from the furnace when it cools to below 100℃. Polishing: Mechanical grinding was performed sequentially using 400#, 800#, 1500#, and 3000# sandpaper, followed by polishing with diamond polishing paste (W3.5, W1.0, W0.5). Finally, electrolytic polishing was performed (the electrolyte was a phosphoric acid-sulfuric acid-chromic acid system, the main components of which included 650 mL / L of phosphoric acid (85%), 225 mL / L of sulfuric acid (98%), 85 g / L of chromic anhydride, 5 g / L of OP-10, and deionized water was added to 1L; the electrolysis conditions were 60℃, current density 15A / dm², and time 5min) until a mirror finish was achieved (polishing brightness 12000 mesh, polishing rate 90%, reaching the SPIA2 quality level), resulting in improved mold steel.

[0026] The rare earth elements in Examples 1-3 were all composed of Ce and La in a weight ratio of 7:3.

[0027] Example 1 Chemical composition of improved mold steel with high mirror polishing properties: C: 0.30%; Si: 0.71%; Mn: 0.12%; Cr: 4.50%; Mo: 3.02%; V: 0.61%; P: 0.012%; S: 0.002%; Synergistic elements (F 0.008%, rare earth 0.035%, Nb 0.03%, B 0.002%): The balance consists of Fe and other unavoidable impurities.

[0028] Example 2 Chemical composition of improved mold steel with high mirror polishing properties: C: 0.39%; Si: 0.90%; Mn: 0.40%; Cr: 5.10%; Mo: 1.35%; V: 0.50%; P: 0.012%; S: 0.002%; Enhancing elements (F 0.01%, rare earth 0.033%, Nb 0.037%, B 0.0023%). The balance consists of Fe and other unavoidable impurities.

[0029] Example 3 Chemical composition of improved mold steel with high mirror polishing properties: C: 0.42%; Si: 1.20%; Mn: 0.53%; Cr: 5.50%; Mo: 2.01%; V: 0.30%; P: 0.012%; S: 0.002%; Enhancing elements (F 0.012%, rare earth 0.04%, Nb 0.035%, B 0.003%). The balance consists of Fe and other unavoidable impurities.

[0030] Comparative Example

[0031] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the synergistic element is only rare earth. Specifically as follows: Chemical composition of improved mold steel with high mirror polishing properties: C: 0.30%; Si: 0.71%; Mn: 0.12%; Cr: 4.50%; Mo: 3.02%; V: 0.61%; P: 0.012%; S: 0.002%; Enhancer element (0.075% rare earth); The balance consists of Fe and other unavoidable impurities.

[0032] The production process for adding synergistic elements is as follows: Addition of enhancing elements: Cerium oxide and lanthanum oxide are injected into the molten steel through the silo in the form of nitrogen gas with a carrier gas pressure of 0.4 MPa and a blowing time of 8 min. During the blowing process, the bottom blowing argon gas flow rate is maintained at 150 NL / min. After the blowing is completed, the mixture is allowed to stand for 5 min to obtain mixture B.

[0033] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the synergistic element is only Nb.

[0034] Chemical composition (mass fraction) of improved mold steel with high mirror polishing properties: C: 0.30%; Si: 0.71%; Mn: 0.12%; Cr: 4.50%; Mo: 3.02%; V: 0.61%; P: 0.012%; S: 0.002%; Synergistic element (Nb 0.075%); The balance is Fe and other inevitable impurities.

[0035] The production process for adding the synergistic element is as follows: Adding the synergistic element: Add ferroniobium powder (niobium content 65 wt%, average particle size 0.001 mm) to the molten steel at a feeding rate of 3 kg / min. After adding, heat it up to 1620 °C by electrifying, and keep it warm for 15 min to fully dissolve and homogenize the elements, obtaining mixture B.

[0036] Comparative Example 3 The difference between Comparative Example 3 and Comparative Example 2 is that the synergistic element in the chemical composition of the improved die steel with high mirror polishing characteristics consists of Nb and B, where Nb is 0.067% and B is 0.008%.

[0037] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that in the chemical composition of the improved die steel with high mirror polishing characteristics, there is no synergistic element, the mass fraction of Cr is 0.075% + 4.50%, and there is no step 2). The mixture A obtained in step 1) is directly used in step (3).

[0038] Performance testing Cut the die steels obtained in Examples 1 - 3 and Comparative Examples 1 - 2 into samples for the following experiments.

[0039] 1) Thermal expansion performance test standard: Refer to GB / T4339 - 2020 "Determination of Thermal Expansion Characteristics of Metallic Materials"; Test parameters: Test temperature range 20 - 400 °C, heating rate 5 °C / min, holding time 10 min, test the linear expansion coefficient; Qualified judgment: When the thermal expansion coefficient ≤ 12.2 * 10 -6 m / m•K, it is recorded as qualified.

[0040] 2) Thermal conductivity Execute in accordance with GB / T22588 - 2008 "Determination of Thermal Conductivity and Thermal Diffusivity of Metallic Materials by the Flash Method"; The diameter of the specimen is 10 mm, the thickness is 5.0 mm, and both sides are parallel and flat; Under the argon protection atmosphere, test the thermal diffusivity of the specimen at three temperature points of 25 °C (room temperature reference), 100 °C, and 200 °C respectively, and calculate the corresponding thermal conductivity; When the measured values of the thermal conductivity are all ≥ 30 W / (m·K), it is judged as qualified.

[0041] 3) Cleanliness Test standard: GB / T10561-2005 "Determination of Non-metallic Inclusion Content in Steel - Standard Rating Chart Microscopic Examination Method"; Test method: The metallographic microscope method was used to observe the entire polished surface at 100x magnification. The field of view with the most serious inclusions was selected, and then compared with the standard rating chart at 500x magnification. Sampling requirements: Take one longitudinal sample from the center of the steel cross-section at half the radius from the edge and from the center. The inspection area of ​​each sample is 200 mm². Randomly select 10 fields of view for statistical analysis. Acceptance criteria: For categories A, B, and C, both fine and coarse inclusions must be ≤0.5 grade; for category D (spherical oxides), both fine and coarse inclusions must be ≤1.0 grade.

[0042] 4) Impact toughness Test standard: Refer to GB / T229-2020 "Charpy Impact Test Method for Metallic Materials"; Test parameters: U-notch specimens with a notch depth of 2mm, pendulum impact energy of 300J, tested at room temperature; Pass / Fail criteria: Impact toughness ≥120J / cm², recorded as pass.

[0043] 5) Corrosion resistance Test standard: Refer to GB / T10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test"; Test parameters: Neutral salt spray (NSS) test was adopted. The test solution was 5% (mass fraction) NaCl aqueous solution with pH value 7, test chamber temperature 37℃, salt spray deposition rate 2mL / (80cm²・h), and the sample was placed vertically in the test chamber at a 30° angle to the salt spray direction. Acceptance criteria: If the salt spray test time is ≥120h and there are no corrosion phenomena such as rust, pitting, discoloration, or blistering on the sample surface, it is considered to have passed the corrosion resistance test.

[0044] If all of the above 1)-5) are qualified, the basic performance is considered qualified.

[0045] 6) Anti-fouling performance Test standard: Refer to GB / T31402-2015 "Test method for stain resistance of plastic surfaces"); Test parameters: 6-1. Take contaminants such as engine oil, isopropanol, and graphite emulsion (Hongda MD-2) and test them using the contact angle method. The amount of contaminant droplet is 5μL, and the contact angle is tested. Pass / fail judgment: When the contact angle of all contaminants is ≥120°, it is recorded as qualified. 6-2. The mold steel industrial mixed dust is selected, consisting of 5 wt% cast iron powder (average particle size of 10 μm, ground from the improved mold steel of Example 1) and 95 wt% environmental dust (ISO 9001). 12103-1 Standard dust (A1 type experimental dust) simulates the composition of actual dust; the sample (50cm wide, 50cm long, 5mm high) is placed in a dust settling chamber (forming a 15° angle with the horizontal plane), with a dust settling amount of 50g / m² and a settling height of 30cm. After settling for 10min, the sample is placed on a vibration table with a frequency of 5Hz and an amplitude of 5mm (forming a 15° angle with the table surface) and gently vibrated for 30s. The mass of residual dust on the sample surface is accurately weighed using an electronic balance; qualification criteria: dust adhesion rate ≤2%, is considered qualified (the lower the adhesion rate, the better the dust adhesion resistance); dust adhesion rate calculation formula: adhesion rate (%) = (mass of residual dust on the sample surface / initial dust mass settling to the sample surface) × 100%; When both the pollutant contact angle and dust adhesion rate meet the requirements, it is recorded as antifouling performance.

[0046] 7) Long-term stability Testing standard: Refer to GB / T30556-2014 "Metallic Materials - Temperature and Humidity Cycling Test Method"; Sample dimensions: 50cm wide, 50cm long, 5mm high Test parameters: A cold and hot cycle aging test was adopted, with the cycle conditions being -40℃×2h→room temperature 25℃×1h→400℃×2h, a single cycle period of 5h, and a total of 100 cycles; after the test, the anti-fouling performance (dust adhesion rate) of the sample was retested according to the test standards and parameters in 6) above; and the dimensions of the sample before and after the aging test were tested, accurate to 0.001mm.

[0047] Performance retention rate calculation formula: Dust adhesion rate retention rate (%) = (performance value after aging / performance value before aging) × 100%, take the average value; Dimension retention rate (%) = 1 - (|performance value after aging - performance value before aging| / performance value before aging) × 100%, take the average value.

[0048] Retention rate grades: 98%≤S grade≤100%, 95%≤A grade<98%; 92%≤B grade<95%; 90%≤C grade<92%; D grade≤90%.

[0049] The specific experimental data are shown in Table 1. Table 1. Experimental data of Examples 1-3 and Comparative Examples 1-2 Combining Examples 1-3 of this application with Comparative Examples 1-4 and Table 1 above, it can be seen that: In terms of basic performance (coefficient of thermal expansion ≤ 12.2*10), -6m / m•K, thermal conductivity ≥30W / (m・K), fine and coarse inclusions of Class A, B, and C are all ≤0.5 grade; fine and coarse inclusions of Class D (spherical oxides) are all ≤1.0 grade, impact toughness ≥120J / cm², salt spray test time ≥120h, pollutant contact angle ≥130°, dust adhesion rate ≤2%), Examples 1-3 are all qualified, while Comparative Examples 1-2 and Comparative Example 4 are all unqualified.

[0050] In terms of long-term stability (cold and hot cycling aging test, cycling conditions: -40℃×2h→room temperature 25℃×1h→400℃×2h, single cycle period 5h, cumulative cycle 100 times), the size retention rate and dust adhesion rate retention rates of Examples 1-3 reached Grade S and Grade A, respectively. In contrast, among Comparative Examples 1-4, the size retention rate of Comparative Example 1 (the synergistic element is only rare earth) decreased to Grade A and the dust adhesion rate decreased to Grade C; the size retention rate of Comparative Example 2 (the synergistic element is only Nb) was Grade A and the dust adhesion rate was Grade C; the size retention rate of Comparative Example 3 (the synergistic element is composed of Nb and B) was Grade A and the dust adhesion rate was Grade C; and the size retention rate of Comparative Example 4 (no synergistic element, and the mass fraction of Cr is 0.075%+4.50%) was Grade B and the dust adhesion rate was Grade D.

[0051] In summary, it can be seen that when the mold steel of this application contains four components, F, rare earth, Nb and B, it plays a synergistic role. In addition to obtaining the basic properties of the improved mold steel (high mirror polishing characteristics, low coefficient of thermal expansion, corrosion resistance, etc.), it further improves the dimensional stability, anti-fouling stability and anti-fouling performance of the mold after long-term use (alternating hot and cold).

[0052] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. An improved die steel with high mirror polishing properties, characterized in that, From the following mass fractions of raw materials composition: C：0.30-0.42%； Si: 0.71-1.20%; Mn: 0.12-0.53%; Cr：4.50-5.50%； Mo: 2.01-3.02%; V：0.30-0.61%； P：≤0.012%； S：≤0.002%； Synergistic element: 0.01-0.10% The balance consists of Fe and other unavoidable impurities; The enhancing elements include: F 0.008-0.012%, rare earth 0.03-0.035%, Nb 0.03-0.04%, and B 0.002-0.003%.

2. The improved mold steel with high mirror polishing properties according to claim 1, characterized in that: The rare earth element is a mixed rare earth element, which includes Ce and La.

3. The improved mold steel with high mirror polishing properties according to claim 1, characterized in that: The mass fraction ratio of Ce to La is 7:

3.

4. A method for preparing an improved mold steel with high mirror polishing properties as described in any one of claims 1-3, characterized in that, Includes the following steps: 1) After mixing all raw materials except for the synergistic elements evenly, proceed with roughing and refining in sequence to form mixture A; 2) Addition of synergistic elements: According to the mass fraction of the formula, add rare earth fluoride composite agent, mix evenly, then add nano boron iron alloy powder, mix evenly, then add niobium iron powder, mix evenly, to obtain mixture B; 3) Then, the process of casting, forging, quenching, tempering, stress relief, and polishing is carried out in sequence to obtain the improved mold steel.

5. The method for preparing an improved mold steel with high mirror polishing properties according to claim 4, characterized in that: The quenching process is carried out at a temperature of 1020-1050℃, and the cooling method is either vacuum quenching or oil quenching.

6. The method for preparing an improved mold steel with high mirror polishing properties according to claim 4, characterized in that: The temperature of the tempering process is ≤500℃.

7. The method for preparing an improved mold steel with high mirror polishing properties according to claim 4, characterized in that: The stress-relief temperature is <300℃, and the cooling method is: furnace cooling ≤500℃ air cooling.

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