High-polishing mirror surface die steel and preparation process thereof

Abstract

The invention discloses high-polishing mirror surface die steel and a preparation process thereof, and belongs to the field of die steel materials. The steel is characterized by comprising the following chemical components in percentage by weight: 15.5-16.0% of Cr, 0.95-1.10% of V and 0.03-0.05% of Ce-La composite rare earth in a specific ratio, and the contents of S and O are strictly controlled. Through vacuum induction melting and electroslag remelting duplex smelting, multidirectional forging and composite heat treatment including subzero treatment and three-stage gradient tempering, an ideal structure which takes high dislocation martensite as a matrix and dispersively distributes nanoscale (Cr, Fe, V) 7C3 carbide and superfine spherical rare earth oxysulfide is finally obtained. After precise grinding, electrolytic polishing and chemical mechanical polishing, the die steel can achieve the ultrahigh mirror surface effect that the surface roughness Ra is smaller than or equal to 0.008 micron while maintaining the high hardness of 52-56 HRC and excellent wear resistance, and is suitable for high-end optical and exterior part dies.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of special alloy steel, and particularly relates to a high-polished mirror surface die steel and a preparation process thereof. BACKGROUND

[0002] The high-polished mirror surface die steel is required to have extremely high purity, excellent hardness and toughness matching, outstanding corrosion resistance and unbeatable polishing performance, so as to achieve a mirror surface effect of Ra ≤ 0.010 μm or even higher. Although the mainstream mirror surface die steels such as S136 of ASSAB in Sweden and NAK80 of Daido in Japan have excellent performance, there are still the following common problems: 1. Polishing performance bottleneck: Non-metallic inclusions (such as Al2O3, silicate, etc.) and uneven distribution of carbide size exist in traditional steels, which are easy to form defects such as “pinholes” and “orange peel” in the polishing process, and become the key factors restricting the further improvement of polishing grade.

[0003] 2. Conflict between corrosion resistance and hardness: Increasing the chromium content to improve corrosion resistance may lead to the precipitation of coarse M7C3 type carbides, which damages the toughness and polishing performance; and too low carbon content is difficult to ensure the hardness and wear resistance of the matrix.

[0004] 3. Insufficient dimensional stability: In the long-term service or heat treatment process, due to excessive residual austenite or insufficient release of organizational stress, the size of the die changes slightly, which affects precise forming.

[0005] The existing technology improves the polishing performance by optimizing heat treatment, but does not fundamentally solve the problem of inclusion and carbide control; some use electroslag remelting to improve purity, but the synergistic control of carbide morphology and distribution is insufficient, and the cost is high. SUMMARY

[0006] The present application aims to overcome the deficiencies of the prior art and provide a high-polished mirror surface die steel and a preparation process thereof. Through innovative alloy composition design and synergistic micro-alloying, combined with special heat treatment and surface treatment process, the high-polished mirror surface die steel can realize ultra-high mirror polishing effect of Ra ≤ 0.008 μm while ensuring high hardness (HRC 54-58) and excellent corrosion resistance, and has excellent dimensional stability and wear resistance.

[0007] To solve the above technical problems, the application provides a high-polishing mirror surface die steel, which comprises the following chemical components in percentage by weight: C: 0.30-0.33%, Si: 0.15-0.25%, Mn: 0.40-0.50%, Cr: 15.5-16.0%, Mo: 1.40-1.60%, V: 0.95-1.10%, Nb: 0.08-0.12%, B: 0.0015-0.0025%, REM: 0.03-0.05%, P: ≤0.010%, S: ≤0.001%, O: ≤0.0008%, N: ≤0.006%, and the balance of Fe and inevitable impurities. The REM is a composite rare earth of cerium (Ce) and lanthanum (La), and the weight ratio of cerium (Ce) to lanthanum (La) is (1.5±0.1):1. The microstructure of the die steel is as follows: a high-dislocation-density tempered lath martensite as a matrix, and (Cr, Fe, V)7C3 type composite carbides with an average size of 80-150 nm dispersedly distributed on the matrix, the number density of the carbides is (3-5)×10 4 / μm 2 The REM exists in the form of spherical Ce-La-O-S composite oxysulfides with an average size of ≤0.5 μm, and is mainly distributed on the prior austenite grain boundaries and the martensite lath boundaries. The working surface roughness Ra of the die steel after composite surface finishing is ≤0.008 μm.

[0008] Preferably, the matrix hardness of the die steel is 52-56 HRC, and the microhardness of the (Cr, Fe, V)7C3 type composite carbides is 15-25% higher than the matrix microhardness.

[0009] A process for preparing the high-polishing mirror surface die steel comprises the following steps: S1, ultra-pure smelting: a vacuum induction melting (VIM) and electroslag remelting (ESR) double process is adopted, the electroslag remelting uses CaF2-Al2O3-CaO-MgO four-element pre-melted slag with a basicity (CaO / SiO2 mass ratio) of 1.8-2.0, and the remelting speed is controlled to be 4.5-5.5 kg / min; S2, forging and multi-stage composite heat treatment: the electroslag remelted ingot is subjected to three-up-three-down multi-directional forging, and the forging ratio is ≥8; then the steel is subjected to spheroidizing annealing, salt bath quenching, deep cryogenic treatment and three-stage gradient tempering in sequence; S3, composite surface finishing: the working surface of the steel after heat treatment is subjected to precise grinding, electrolytic polishing and chemical mechanical polishing (CMP) in sequence.

[0010] Preferably, the spheroidizing annealing process in step S2 is heating to 870±5℃ at a rate of ≤100℃ / h, holding for 4±0.5 hours, then slowly cooling to 690±5℃ at a rate of 12±2℃ / h, holding for 10±1 hours, and finally furnace cooling to below 450℃ and discharging.

[0011] Preferably, the salt bath quenching in step S2 is austenitizing at 1040±5℃ in a salt bath furnace, holding for a time calculated according to 1.0±0.1min / mm, and then oil quenching to room temperature; and the cryogenic treatment is placing the workpiece in an environment of -90±5℃ within 1 hour after quenching and holding for 3±0.5 hours, and then naturally recovering to room temperature.

[0012] Preferably, the three-stage gradient tempering in step S2 is specifically: First tempering: holding at 510±5℃ for 2.5±0.3 hours, and air cooling to room temperature; Second tempering: holding at 490±5℃ for 2.5±0.3 hours, and air cooling to room temperature; Third tempering: holding at 470±5℃ for 2.5±0.3 hours, and air cooling to room temperature.

[0013] Preferably, the electrolytic polishing in step S3 uses an electrolyte of a mixture of phosphoric acid, sulfuric acid and glycerol, with a volume ratio of phosphoric acid:sulfuric acid:glycerol=7:2:1; and the polishing conditions are: liquid temperature 35±2℃, current density 20±2A / dm², and duration 6±1 minutes.

[0014] Preferably, the chemical mechanical polishing (CMP) in step S3 uses a weakly alkaline nano cerium oxide polishing liquid, wherein the average particle size of nano cerium oxide (CeO2) is 40±5nm, and the pH value of the polishing liquid is 9.0±0.2; and the polishing process parameters are: polishing pressure 15±3kPa, polishing disc rotation speed 70±5rpm, and polishing time 25±3 minutes.

[0015] Preferably, in the starting stage of the electroslag remelting in step S1, the arc striking is performed using the intermediate alloy of the REM, and the weight ratio of Ce to La in the intermediate alloy is consistent with the final composition requirement. Advantages

[0016] 1. Component system innovation: By precisely regulating the contents of C, Cr, Mo, V, Nb, and combining with the control of extremely low harmful elements (P, S, O, N), an alloy system that can form high-density nanoscale (Cr, Fe, V)7C3 type complex carbide is designed, which lays the foundation for high hardness, high wear resistance and excellent polishing; The specific proportion (Ce: La ≈ 1.5: 1) of composite rare earth (REM) is innovatively introduced, and the specific process is used to form ultra-fine spherical sulfide, which not only effectively purifies the liquid steel and refines the grains, but also significantly improves the toughness and high temperature stability of the material through the pinning effect of the grain boundary; The content of boron (B) is controlled in the extremely low range (0.0015-0.0025%), which cooperates with rare earth elements to further strengthen the grain boundary and avoid the formation of brittle phases that cause polishing defects.

[0017] 2. Organization and performance innovation: The ideal microstructure of "high dislocation density tempered plate martensite matrix + ultra-high density nanoscale (Cr, Fe, V)7C3 complex carbide dispersed distribution" is obtained, which realizes the perfect combination of high toughness of the matrix and second phase strengthening (carbide hardness is 15-25% higher than the matrix), while ensuring high hardness (52-56HRC), It gives the material excellent plastic deformation resistance and uniform polishing performance; Through process control, harmful REM inclusions are converted into small size (≤0.5μm), round shape spherical composite sulfide, and mainly distributed in the grain boundary, avoiding the damage of large irregular inclusions to the polished surface, which is the key to realize Ra≤0.008μm super mirror polishing.

[0018] 3. Preparation process innovation: The "VIM+ESR double" ultra-pure smelting and specific basicity slag remelting process is adopted, which reduces the content of gas and impurity elements to a very low level from the source, ensuring the cleanliness of the material; The multi-stage composite heat treatment process of "three upsetting three pulling multi-directional forging + specific spheroidizing annealing + salt bath quenching + cryogenic treatment + three-stage gradient tempering" is designed, which effectively breaks the as-cast structure, obtains uniform and fine carbides, promotes the transformation of residual austenite, stabilizes the structure and eliminates internal stress, providing guarantee for the final performance; The composite surface finishing technology of "precision grinding + electrolytic polishing + chemical mechanical polishing (CMP)" is adopted. Especially the CMP process using weak alkaline nano cerium oxide polishing solution can realize atomic scale material removal, thereby obtaining a nearly perfect mirror surface.

[0019] 4. Synergy between composite rare earth (REM) and ultrapure smelting and electroslag remelting start-up processes: This achieves synergistic effects in deep purification of molten steel, optimized control of inclusion morphology throughout the entire process, and precise fine-tuning of composition. Ultrapure smelting (VIM+ESR) provides a clean environment with low oxygen and sulfur for REM to exert its beneficial effects. During the ESR start-up stage, an intermediate alloy with a specific Ce / La ratio is used for arc initiation, ensuring that REM is uniformly added at the initial stage of remelting, fully leveraging its deoxidation, desulfurization, and microalloying functions. The resulting ultrafine spherical Ce-La-OS composite not only fixes residual trace amounts of O and S, preventing them from forming harmful inclusions on their own, but also transforms normally harmful inclusions into beneficial microstructure-regulating "pinning points" through its fine spherical morphology and grain boundary distribution, synergistically improving the purity, toughness, and polishing performance of the material.

[0020] 5. Synergy between nanoscale (Cr,Fe,V)7C3 carbides and multi-stage composite heat treatment: Synergy achieves a unity of high strength, high wear resistance, and excellent polishability. Specific alloy compositions (such as V and Nb) determine the type and potential size of precipitated carbides. Through a process of "rapid and uniform austenitization in a salt bath + oil quenching + deep cryogenic treatment," supersaturated martensite with high dislocation density is obtained, providing sufficient nucleation sites for the large-scale dispersed precipitation of carbides during subsequent tempering. Three-stage gradient tempering is key. The first tempering at a relatively high temperature promotes the uniform precipitation and initial growth of a large number of (Cr,Fe,V)7C3 carbides at the nanoscale (80-150 nm). The subsequent two temperings with gradually decreasing temperatures further spheroidize and stabilize the carbides, and release internal stresses at different stages. Ultimately, these carbides with extremely high density (3-5 × 10⁻⁶) exhibit excellent wear resistance and polishability. 4 per μm 2 Nano-carbide with a hardness higher than that of the matrix and uniform size is uniformly embedded in the tough martensitic matrix as a "hard phase". During polishing, it can achieve synchronous and uniform micro-removal of the matrix and hard phase, avoiding polishing defects such as "orange peel" or "pinhole" caused by uneven structure.

[0021] 6. Electrolytic polishing and chemical mechanical polishing (CMP) in composite surface finishing: realizing the step-by-step precision surface construction from macro to micro, from micron to nanometer and even atomic scale, finally reaching the super mirror level. Precision grinding first removes macro unevenness, electrolytic polishing as an intermediate bridge, using the principle of anode dissolution, quickly and uniformly removes grinding marks and micro convexity in a non-contact manner, significantly reduces surface roughness, and does not introduce new mechanical stress or scratches. On this basis, chemical mechanical polishing (CMP) plays a final finishing role. Its alkaline nano CeO2 polishing solution has a weak chemical reaction with the steel surface, forming a soft layer that is easy to remove, and at the same time, through the mechanical friction of nano abrasive, the layer is precisely removed. The synergy of this chemical and mechanical action realizes atomic-level material migration, which can effectively eliminate the micro waviness that may be left after electrolytic polishing, and finally obtain a super smooth mirror surface with a surface roughness Ra≤0.008μm, meeting the surface requirements of the highest level of optical products, high-end appearance parts and other molds. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a schematic diagram of the microstructure of the high polishing mirror mold steel of the present application.

[0023] Figure 2 It is a preparation process flow chart of the high polishing mirror mold steel of the present application.

[0024] Figure 3 It is a columnar graph for comparing the Rockwell hardness of Example 2 and the comparative example.

[0025] Figure 4 It is a columnar graph for comparing the impact energy of Example 2 and the comparative example.

[0026] Figure 5 It is a columnar graph for comparing the relative wear resistance of Example 2 and the comparative example.

[0027] Figure 6 It is a columnar graph for comparing the surface roughness of Example 2 and the comparative example. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0029] Example 1

[0030] Composition (wt%): C: 0.31%, Si: 0.15%, Mn: 0.40%, Cr: 15.5%, Mo: 1.40%, V: 0.95%, Nb: 0.08%, B: 0.0015%, REM: 0.03% (Ce:La = 1.5: 1), P: 0.008%, S: 0.0008%, O: 0.0006%, N: 0.004%, Fe: balance.

[0031] Key parameters of the manufacturing process: S1 Smelting: VIM + ESR double combination, ESR slag system basicity 1.8, melting speed 5.5 kg / min, using REM intermediate alloy arc.

[0032] S2 Heat treatment: three-up three-drawing forging ratio = 8. Spheroidizing annealing: 870°C x 4h, slow cooling to 690°C x 10h. Salt bath quenching: 1040°C x (calculated according to thickness), oil quenching. Cryogenic treatment: -90°C x 3h. Three-stage gradient tempering: 510°C x 2.5h + 490°C x 2.5h + 470°C x 2.5h.

[0033] S3 Surface finishing: After precision grinding, electrolytic polishing (phosphoric acid: sulfuric acid: glycerol = 7:2:1, 35°C, 20 A / dm 2 , 6min), CMP (nano CeO2 polishing solution pH = 9.0, pressure 15kPa, 70rpm, 25min).

[0034] Example 2

[0035] Composition (wt%): C: 0.315%, Si: 0.20%, Mn: 0.45%, Cr: 15.75%, Mo: 1.50%, V: 1.025%, Nb: 0.10%, B: 0.0020%, REM: 0.04% (Ce:La = 1.5: 1), P: 0.005%, S: 0.0005%, O: 0.0005%, N: 0.003%, Fe: balance.

[0036] Key parameters of the manufacturing process: S1 Smelting: same as Example 1, ESR slag system basicity 1.9, melting speed 5.0 kg / min.

[0037] S2 Heat treatment: forging ratio ≥ 10. Spheroidizing annealing: 870°C x 4h, slow cooling to 690°C x 10h. Salt bath quenching: 1040°C x (calculated according to thickness), oil quenching. Cryogenic treatment: -90°C x 3h. Three-stage gradient tempering: 510°C x 2.5h + 490°C x 2.5h + 470°C x 2.5h.

[0038] S3 surface finishing: precision grinding, electrolytic polishing (phosphoric acid: sulfuric acid: glycerol = 7:2:1, 35°C, 20 A / dm 2 , 6 min), CMP (nano-CeO2 polishing solution pH = 9.0, pressure 15 kPa, 70 rpm, 25 min).

[0039] Example 3

[0040] Ingredients (wt%): C: 0.32%, Si: 0.25%, Mn: 0.50%, Cr: 16.0%, Mo: 1.60%, V: 1.10%, Nb: 0.12%, B: 0.0025%, REM: 0.05% (Ce:La = 1.5:1), P: 0.006%, S: 0.0009%, O: 0.0007%, N: 0.005%, Fe: balance.

[0041] Key parameters of the preparation process: S1 smelting: same as Example 1, ESR slag system basicity 2.0, melting rate 4.5 kg / min.

[0042] S2 heat treatment: forging ratio = 12. spheroidizing annealing: 870°C x 4h, slow cooling to 690°C x 10h. salt bath quenching: 1040°C x (calculated according to thickness), oil quenching. cryogenic treatment: -90°C x 3h. three-stage gradient tempering: 510°C x 2.5h + 490°C x 2.5h + 470°C x 2.5h.

[0043] S3 surface finishing: precision grinding, electrolytic polishing (phosphoric acid: sulfuric acid: glycerol = 7:2:1, 35°C, 20 A / dm 2 , 6 min), CMP (nano-CeO2 polishing solution pH = 9.0, pressure 15 kPa, 70 rpm, 25 min).

[0044] Comparative Example 1

[0045] Ingredients: the ingredients are basically the same as Example 2, but no REM is added, and the S content is slightly higher, at 0.002%.

[0046] Process: smelting uses conventional VIM + ESR, but no REM arc and addition. Heat treatment and surface finishing are the same as Example 2.

[0047] Comparative Example 2

[0048] An alloy ingredient design similar to Example 2 is used, but a conventional electric arc furnace (EAF) smelting + argon decarburization (AOD) process is used, without ESR.

[0049] Process: after smelting, direct forging, heat treatment same as Example 2. Surface finishing same as Example 2.

[0050] Comparative Example 3

[0051] Composition: Same as Example 2.

[0052] Process: The melting process is same as Example 2. The heat treatment is changed to: after salt bath quenching (1040°C), directly proceed single 510°C x 3 hours tempering, omit deep cryogenic treatment and second, third tempering.

[0053] Comparative Example 4

[0054] Composition and heat treatment are same as Example 2.

[0055] Process: The surface finishing only proceed precision grinding and diamond paste mechanical polishing, omit electrolytic polishing and chemical mechanical polishing (CMP).

[0056] Comparative Example 5

[0057] Composition: Same as Example 2.

[0058] Process: The melting process is same as Example 2. The heat treatment process is changed to conventional process: after salt bath quenching (1040°C), directly proceed single 510°C x 7.5 hours tempering, omit deep cryogenic treatment and gradient tempering.

[0059] All the samples of above examples and comparative examples are tested for key properties, hardness by Rockwell hardness C scale, cross section multi-point average value; impact energy by Charpy V-notch impact test; relative wear resistance by ball-disk wear test, under the same load, rotation speed, with the wear volume of Example 2 as benchmark 1.0 for calculation; surface roughness Ra by using white light interferometer measurement; carbide characteristics by transmission electron microscopy (TEM) image statistics; REM inclusions by using scanning electron microscope with energy spectrum (EDS) observation, statistics of maximum size and typical morphology; polishing surface defects by using optical microscope observation. The specific data results are shown in the table below:

[0060] Test results: The data of Examples 1-3 show that, with the component and process adjustment within the scope of the claims, the properties (hardness 54-56 HRC, impact energy 22-25 J, Ra≤0.008 μm) can all reach excellent and balanced levels. This simultaneous realization of high hardness, good toughness, ultra-high wear resistance and ultra-mirror polishing effect is unattainable by a single technical feature. Example 2 (containing REM, S very low) and Comparative Example 1 (no REM, S = 0.002%) have a huge difference in performance under the same heat treatment and finishing process. The REM inclusions of Comparative Example 5 are irregular sulfides with a maximum size of 3.0 μm, resulting in a polished surface roughness Ra(0.0220 μm) that is about 3.5 times worse than that of Example 2 (0.0062 μm), and the impact energy (17.5 J) is significantly reduced, which directly proves that the addition of REM alone is not enough to achieve the effect, and it must be converted into ≤0.5 μm spherical composite oxysulfide through the component design (very low S) and process control (ESR plus REM arc striking) of the present application to eliminate the polishing defect source and improve toughness. The difference in microstructure and properties between Example 2 (three-stage gradient tempering + cryogenic treatment) and Comparative Example 5 (single long-time tempering) is that the average size of the carbides in Comparative Example 5 is larger (165 nm) and the number density is lower (2.2 x 10 4 2 ​), the distribution is more uneven, which directly leads to its wear resistance of only 80% of that of Example 2, the impact energy decreases by 18%, and the surface homogeneity after polishing decreases (Ra0.0105 μm, orange peel tendency), which verifies that the three-stage gradient tempering and cryogenic treatment are indispensable for precipitating high-density, fine spherical nanocarbide and fully stabilizing the microstructure and releasing stress, and are the core process for realizing high wear resistance, high toughness and uniform polishing. Example 2 (VIM+ESR) is compared with Comparative Example 2 (EAF+AOD), and Comparative Example 2 is inferior in hardness, toughness, wear resistance and polishing due to low purity, coarse microstructure, low number density of carbides and the presence of large brittle inclusions, especially the impact energy and surface quality are the worst, which proves that the VIM+ESR double process and the specific slag system are the necessary prerequisite for realizing extremely low gas and impurity content and obtaining clean and uniform microstructure. Example 2 (grinding+electrolytic polishing+CMP) is compared with Comparative Example 4 (only grinding+mechanical polishing), and although Comparative Example 4 has the same excellent matrix microstructure (carbide characteristics, hardness and toughness) as Example 2, its surface roughness Ra(0.0150 μm) is much higher than that of Example 2, and there are mechanical scratches, which strongly shows that the excellent matrix microstructure (internal cause) provides the possibility for super mirror polishing, but it must be converted into reality through the specific composite finishing process (external cause) of "electrolytic polishing+CMP", and electrolytic polishing realizes stress-free uniform removal, and CMP realizes atomic-level flatness, both of which are indispensable. The performance of Comparative Example 3 (only change the heat treatment) and Comparative Example 1 (only change the composition) appears "short board effect", and the present application realizes high hardness and high toughness, high wear resistance and super mirror polishing at the same time through the synergy of "specific REM addition and morphology control" + "ideal nanocarbide obtained by multi-stage heat treatment" + "ultra-pure smelting", for example, Example 2 has a hardness of 55HRC, an impact energy of 25J, and realizes super mirror with Ra0.0062 μm, and the improvement of such comprehensive performance is significant and non-obvious.

[0061] The above-described examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as limiting the scope of the patent of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A high-polish mirror-finish mold steel, characterized in that, The chemical composition, by weight percentage, includes: C: 0.30-0.33%, Si: 0.15-0.25%, Mn: 0.40-0.50%, Cr: 15.5-16.0%, Mo: 1.40-1.60%, V: 0.95-1.10%, Nb: 0.08-0.12%, B: 0.0015-0.0025%, REM: 0.03-0.05%, P: ≤0.010%, S: ≤0.001%, O: ≤0.0008%, N: ≤0.006%, with the balance being Fe and unavoidable impurities; Wherein, REM is a composite rare earth element of cerium (Ce) and lanthanum (La), and the weight ratio of cerium (Ce) to lanthanum (La) is (1.5±0.1):1; The microstructure of the mold steel is as follows: tempered lath martensite with high dislocation density serves as the matrix, on which (Cr,Fe,V)7C3 type composite carbides with an average size of 80-150 nm are dispersedly distributed, and the number density of the carbides is (3-5)×10⁻⁶. 4 The REM exists in the form of spherical Ce-La-OS composite oxysulfides with an average size ≤0.5μm, and is mainly distributed at the original austenite grain boundaries and martensite lath boundaries. After composite surface finishing, the surface roughness Ra of the mold steel is ≤0.008μm.

2. The high-polish mirror-finish mold steel according to claim 1, characterized in that, The matrix hardness of the mold steel is 52-56 HRC, and the microhardness of the (Cr,Fe,V)7C3 type composite carbide is 15-25% higher than that of the matrix.

3. A process for preparing the high-polish mirror-finish mold steel as described in claim 1 or 2, characterized in that, Includes the following steps: S1. Ultra-pure smelting: A dual process of vacuum induction melting (VIM) and electroslag remelting (ESR) is adopted. The electroslag remelting uses a CaF2-Al2O3-CaO-MgO quaternary pre-melted slag with a basicity (CaO / SiO2 mass ratio) of 1.8-2.0, and the remelting rate is controlled at 4.5-5.5 kg / min. S2. Forging and multi-stage composite heat treatment: The electroslag remelted ingot is subjected to three upsetting and three drawing multi-directional forging with a forging ratio ≥8; then it is subjected to spheroidizing annealing, salt bath quenching, deep cryogenic treatment and three-stage gradient tempering in sequence. S3. Composite Surface Finishing: The working surface of the heat-treated steel is then subjected to precision grinding, electrolytic polishing, and chemical mechanical polishing (CMP) in sequence.

4. The process according to claim 3, characterized in that, The spheroidizing annealing process described in step S2 is as follows: heating to 870±5℃ at a rate of ≤100℃ / h and holding for 4±0.5 hours; then slowly cooling to 690±5℃ at a rate of 12±2℃ / h and holding for 10±1 hours; finally furnace cooling to below 450℃ and then unloading from the furnace.

5. The process according to claim 3, characterized in that, The salt bath quenching in step S2 is as follows: austenitization is performed in a salt bath furnace at 1040±5℃, and the holding time is calculated at 1.0±0.1min / mm, followed by oil quenching to room temperature; the cryogenic treatment is as follows: within 1 hour after quenching, the workpiece is placed in an environment of -90±5℃ for 3±0.5 hours, and then naturally restored to room temperature.

6. The process according to claim 3, characterized in that, The three-level gradient annealing mentioned in step S2 specifically refers to: First tempering: Hold at 510±5℃ for 2.5±0.3 hours, then air cool to room temperature; Second tempering: Hold at 490±5℃ for 2.5±0.3 hours, then air cool to room temperature; Third tempering: Hold at 470±5℃ for 2.5±0.3 hours, then air cool to room temperature.

7. The process according to claim 3, characterized in that, The electrolyte used in step S3 for electrolytic polishing is a mixture of phosphoric acid, sulfuric acid, and glycerol, with a volume ratio of phosphoric acid:sulfuric acid:glycerol = 7:2:1; the polishing conditions are: liquid temperature 35±2℃, current density 20±2A / dm³. 2 The duration is 6 ± 1 minutes.

8. The process according to claim 3, characterized in that, The polishing slurry used in the chemical mechanical polishing (CMP) in step S3 is a weakly alkaline nano-cerium oxide polishing slurry, wherein the average particle size of the nano-cerium oxide (CeO2) is 40±5nm, the pH value of the polishing slurry is 9.0±0.2, and the polishing process parameters are: polishing pressure 15±3kPa, polishing disc rotation speed 70±5rpm, and polishing time 25±3 minutes.

9. The process according to any one of claims 3 to 8, characterized in that, In the start-up stage of electroslag remelting described in step S1, the intermediate alloy of REM is used for arc initiation, and the weight ratio of Ce to La in the intermediate alloy is consistent with the final composition requirements.

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