A large-scale integrated die-casting process special hot work die steel and a preparation method thereof

Abstract

The application discloses a kind of large-scale integrated special hot die steel for die casting and a preparation method thereof, and belongs to the technical field of hot die steel.The chemical composition of the steel includes, by mass percentage: C 0.28-0.35%, La 0.01-0.025%, Ce 0.01-0.02%, Mn 0.4-0.45%, Si 0.1-0.2%, Cr 4.5-5.5%, W 1.50-2.00%, Mo 2.2-2.8%, V 0.8-1.2%, Co 1.0-1.5%, Nb 0.1-0.3%, and the balance is Fe and impurities (S, P≤0.015%).The preparation method includes: vacuum induction melting and electroslag remelting purification, hydrogen burning high-temperature homogenization, three-up-three-down multi-directional forging, spheroidizing annealing and deep cryogenic treatment, multi-stage quenching and double tempering, multi-element co-permeation, surface nanocrystallization by supersonic particle bombardment and plasma boronizing compound process.Through rare earth elements La / Ce refining grain boundary, combined with multi-element alloy strengthening and surface nanocrystallization-boriding synergistic treatment, the hardness, impact toughness and thermal fatigue life of the die steel are significantly improved, and it is suitable for high temperature, high pressure and high wear conditions of large-scale integrated die casting dies.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal materials, in particular to a special hot work die steel for large-scale integrated die casting process and a preparation method thereof, and especially to a die steel material for long-term service under high pressure and high temperature conditions. BACKGROUND

[0002] With the rapid development of new energy vehicles and lightweight structural parts, large-scale integrated die casting technology is widely used due to its high efficiency and high precision. However, the traditional hot work die steel (such as H13 and 8407) is prone to thermal fatigue cracks, high-temperature softening, and oxidation wear when subjected to high-temperature and high-pressure molten metal impact for a long time, resulting in short die life and high maintenance cost.

[0003] In the prior art, molybdenum (Mo) and vanadium (V) elements are added to improve high-temperature strength, or electroslag remelting is used to improve purity, but there are still the following defects: grain boundary weakening at high temperature, insufficient thermal fatigue resistance; low thermal conductivity, leading to local overheating of the die; size effect of large-scale die leading to uneven structure.

[0004] Therefore, there is an urgent need to develop a new type of hot work die steel with high thermal conductivity, thermal fatigue resistance, high-temperature strength, and excellent processing performance. SUMMARY

[0005] The purpose of the present application is to provide a special hot work die steel for large-scale integrated die casting process and a preparation method thereof, to solve the problems of thermal fatigue cracks, high-temperature softening, and oxidation wear of the current hot work die steel when subjected to high-temperature and high-pressure molten metal impact for a long time.

[0006] Technical solution: A special hot work die steel for large-scale integrated die casting, the chemical composition of the molten steel includes, by mass percentage: C: 0.28-0.35%, La: 0.01-0.025%, Ce: 0.01-0.02%, Mn: 0.4-0.45%, Si: 0.1-0.2%, Cr: 4.5-5.5%, W: 1.50-2.00%, Mo: 2.2-2.8%, V: 0.8-1.2%, Co: 1.0-1.5%, Nb: 0.1-0.3%, the balance is Fe and impurities, and the contents of S and P are both ≤0.015%.

[0007] A preparation method of a special hot work die steel for large-scale integrated die casting, comprising the following steps:

[0008] S1: The raw materials are vacuum melted in proportion by vacuum induction melting and electroslag remelting method, and protective slag is added for pouring, and the electrode rod is cooled and annealed into electrode rod, and the electrode rod is annealed and subjected to secondary electroslag remelting, and the annealed steel ingot 1 is obtained;

[0009] S2: After high temperature homogenization of the ingot 1 obtained in step S1, a layer of mixed nano metal powder is spread on the surface, and hydrogen flame is burned for 5 minutes, and then three-up and three-down multi-axle forging is performed to obtain an ingot 2;

[0010] S3: The ingot 2 obtained in step S2 is immediately subjected to cold deformation-spheroidizing annealing and deep cryogenic treatment to obtain an ingot 3;

[0011] S4: The ingot 3 obtained in step S3 is subjected to step quenching and twice tempering treatment to obtain an ingot 4;

[0012] S5: After surface corrosion of the ingot 4 obtained in step S4, multi-element co-permeation is performed to obtain an ingot 5;

[0013] S6: The ingot 5 obtained in step S5 is subjected to surface nanocrystallization by using supersonic particle bombardment technology to obtain an ingot 6;

[0014] S7: The ingot 6 obtained in step S6 is subjected to surface plasma boronizing to obtain a finished product.

[0015] Preferably, in step S1, the pouring is performed by using a cap-bottom pouring method with a protective slag, the mold is demolded after 3 hours of pouring, the electrode rod is annealed by heating at a gradient of 100℃ / h to 900℃ and maintaining for 7 hours, and then the electrode rod is cooled to 300℃ in the furnace at a cooling speed of 50℃ / h; the cap of the electrode rod is removed before secondary current remelting, the oxide skin on the surface of the electrode rod is removed, and the electrode rod is heated to 880℃ at a heating speed of 100℃ / h and maintained for 6 hours, and then cooled to 300℃ in the furnace at a cooling speed of 40℃ / h and discharged; the electroslag remelting is performed by using a CaF2-Al2O3-CaO slag system, the current density is 5-8 A / cm², the remelting rate is 3-5 kg / min, and the purity is improved to the level of ≤0.5% of inclusions.

[0016] Preferably, in step S2, the high temperature homogenization is performed by heating at a heating speed of 90℃ / h to 1250℃ and maintaining for 6-8 hours, the mixed nano metal powder is a mixed nano metal powder of titanium:silver:nickel:iron (w / w)=1:1:2:20, the initial forging temperature of the multi-axle forging is 1150-1180℃, the reheating temperature is 1250℃, the forging ratio is ≥6, and the final forging temperature is ≥900℃.

[0017] Preferably, in step S3, the cold deformation is cold bending deformation, and the deformation amount is 10%-20%; after deformation, heating and maintaining are performed, the heating is performed to 800℃, the spheroidizing annealing is performed by annealing at 800℃ for 4 hours, and then the furnace is cooled to 500℃ and air-cooled; the deep cryogenic treatment is performed by liquid nitrogen treatment at -196℃ for 2 hours.

[0018] Preferably, in step S4, the step of grading quenching is austenitizing at 1020-1050℃, holding for 1h, and then oil cooling to 200℃; the step of primary tempering is tempering at 560℃ for 3h, and then air cooling; the step of secondary tempering is tempering at 520℃ for 2h, and then air cooling.

[0019] Preferably, in step S5, the step of surface corrosion is spraying corrosion with 2M hydrochloric acid ethanol solution for 2min, and then washing with purified water; the step of multi-element co-diffusion is co-diffusion at 520℃ for 6h, using a mixed solution of tetramethyl thiourea, ammonium borate, propionamide and ethanol (W / W) = 1:0.5:6:10 as the co-diffusion agent; the co-diffusion layer thickness is 0.15-0.20mm, and the surface hardness is ≥1100HV.

[0020] Preferably, in step S6, the method of surface nanocrystallization by supersonic particle bombardment technology is as follows: compressed air is used as carrier gas, the compressed air obtains supersonic speed through a Laval nozzle, and the compressed air carries hard corundum particles with a particle size of 1-2μm to the surface of the material ground to a roughness Ra=0.8-1μm by a grinding machine, and due to the high-speed repeated impact of the countless particles on the surface of the material, the surface layer of the material is strongly plastically deformed, leading to surface grain refinement and gradual nanocrystallization. The supersonic particle bombardment metal surface nanocrystallization device has four parts: a supersonic spray gun and an operating manipulator, a worktable, and a parameter monitoring and automatic control system. The key component of the supersonic particle bombardment is the spray gun, which is designed according to the principle of the Laval nozzle. The gas flow enters the narrow throat through the contraction section and then expands to obtain a supersonic speed of 700-1200m / s. The gas source is 10-15MPa high-pressure compressed air. The gas is divided into two paths: one path leads to the powder feeder, carrying the powder into the spray gun; the other path connects the heater and then enters the spray gun. The spray gun can be fixed on the manipulator to realize three-dimensional and six-directional control. The workpiece is placed on the worktable in the operating chamber, which can rotate 360°. The operating chamber is connected to a powder recovery system. After the powder is recovered, it can be reused. Therefore, it is necessary to check whether each fixed bolt is tightened. After the processing is completed, the sample is taken out after cooling to room temperature. The surface is cleaned with alcohol again to wash away some garbage and iron filings remaining on the surface. Finally, the nanocrystallized surface is obtained and placed in a dry environment to prevent surface oxidation.

[0021] Preferably, in step S7, the surface plasmonic boronizing method is as follows: the surface-nanocrystallized sample surface is cleaned with cyclohexane and blown dry, placed in a plasma heat treatment furnace that has been wiped clean, the sealed part is wiped clean with cyclohexane, the furnace cover is covered, the vacuum pump is started, the system is pumped to below 0.01 KPa, the current and voltage are turned on, the arc striking stage is entered, after the arc striking is completed, the gas is started, first the mixed gas of argon: hydrogen = 2-4:1 is introduced, the temperature rising stage is started, the current can be adjusted to be larger at the beginning to improve the temperature rising efficiency, as the temperature rises, the current should be gradually reduced to prevent the temperature from rising too fast, after the temperature rises to 750-800℃ and stabilizes, 1-2% of ethyl borane of the total volume of argon and hydrogen is introduced, and the temperature is kept for 3-5 hours, after the temperature keeping is completed, the current and voltage are turned off, the residual gas in the furnace and pipeline is pumped out, then a certain amount of argon is introduced to cool the sample to room temperature in the protection of argon atmosphere, the surface plasmonic boronizing temperature is 750-800℃, the boronizing time is 3-5 hours, and the boronizing layer thickness is 0.05-0.10mm.

[0022] Advantages:

[0023] 1. By vacuum induction melting and electroslag remelting methods, and using CaF2-Al2O3-CaO slag system for secondary remelting, the purity of the steel is effectively improved, which helps to reduce cracks and fractures caused by inclusions during the use of the mold, thereby significantly improving the service life of the mold.

[0024] 2. Using cold deformation-spheroidizing annealing makes the carbides of the mold steel in spherical distribution, reduces the cutting effect of the carbides on the matrix, improves the toughness and processing performance of the material, and deep cryogenic treatment further refines the grains, enhances the strength and hardness of the mold steel.

[0025] 3. By high-speed particle impact on the surface of the mold steel, the surface is nanocrystallized, and the surface grains are refined to nanoscale. This nanocrystallized surface not only improves the wear resistance of the mold, but also effectively reduces the surface wear and fatigue crack generation of the mold during use.

[0026] 4. A 0.15-0.20mm thick multi-element permeation layer is formed on the surface of the mold steel, and the surface hardness reaches ≥1100HV. The multi-element permeation layer significantly improves the wear resistance and anti-seizure performance of the mold, reduces the adhesion phenomenon between the mold and the die casting material, thereby prolonging the service life of the mold.

[0027] 5. On the basis of nanocrystallization, plasma boronizing treatment is carried out to form a 0.05-0.10mm thick boronizing layer, which further improves the surface hardness and wear resistance of the mold, and enhances the corrosion resistance of the mold, so that it can maintain good performance in high temperature and corrosive environment.

[0028] 6. The mixed nano-metal powder can strengthen the surface, improve the hardness and wear resistance; the nano-particles pin the grain boundaries, combine the multi-directional forging to refine the grains; the silver reduces the forging defects, improves the formability; provides a reaction source for subsequent boronizing and co-permeation, and enhances the performance of the permeation layer. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The surface TEM image of the present application after surface nanocrystallization by supersonic particle bombardment technology.

[0030] Figure 2 The surface crack comparison chart of the thermal fatigue test of the surface nanocrystallization (left) and the surface nanocrystallization (right) for 6000 times.

[0031] Figure 3 The SEM image of the wear surface morphology of the sample after plasma boronizing treatment (left) and without plasma boronizing treatment (right) under a load of 45N.

[0032] Figure 4 The X-ray diffraction chart of the surface multi-element co-permeation and non-co-permeation. DETAILED DESCRIPTION

[0033] In order to make the technical scheme of the present application clearer, the present application will be further described in detail below in combination with the drawings and specific embodiments.

[0034] Embodiment 1

[0035] Chemical composition (mass percent): C: 0.30%, La: 0.015%, Ce: 0.015%, Mn: 0.42%, Si: 0.15%, Cr: 5.0%, W: 1.75%, Mo: 2.5%, V: 1.0%, Co: 1.25%, Nb: 0.2%, the balance of Fe and impurities, and the contents of S and P are both ≤0.015%.

[0036] Preparation method:

[0037] Step S1: adopt the capped mouth bottom pouring method and add protective slag for pouring, and demould after pouring for 3h. Anneal the electrode rod at a gradient of 100℃ / h to 900℃ and keep for 7h, then cool down to 300℃ with the furnace at a cooling speed of 50℃ / h. Remove the electrode rod cap before secondary current remelting, remove the oxide skin on the surface of the electrode rod, heat to 880℃ at a heating speed of 100℃ / h and keep for 6h, then cool down to 300℃ with the furnace at a cooling speed of 40℃ / h and take out. The electroslag remelting adopts CaF2-Al2O3-CaO slag system, the current density is 6A / cm², the remelting rate is 4kg / min, and the purity is improved to ≤0.5 level of inclusions.

[0038] Step S2: homogenization at 90℃ / h heating rate to 1250℃, holding for 7h. Sprinkle a layer of mixed nano metal powder on the surface, hydrogen flame burning for 5min, multi-directional forging initial forging temperature is 1160℃, reheat temperature is 1250℃, forging ratio is ≥6, final forging temperature is ≥900℃.

[0039] Step S3: cold bending deformation of 10%, spheroidizing annealing at 800℃ for 4h, furnace cooling to 500℃ and then air cooling; deep cooling at -196℃ for 2h.

[0040] Step S4: austenitizing at 1030℃ for 1h, oil cooling to 200℃, primary tempering at 560℃ for 3h, air cooling, secondary tempering at 520℃ for 2h, air cooling.

[0041] Step S5: 2M hydrochloric acid ethanol solution spray etching for 2min, pure water rinsing, multi-element co-diffusion at 520℃ for 6h, co-diffusion layer thickness is 0.18mm, surface hardness is ≥1100HV. The X-ray diffraction patterns of the surface multi-element co-diffusion and non-co-diffusion are shown in the attached Figure 4 , from which it can be seen that the X-ray diffraction peaks of the surface multi-element co-diffusion sample are obviously widened compared with the non-co-diffusion sample.

[0042] Step S6: surface nanocrystallization by supersonic particle bombardment technology, using 12MPa high pressure compressed air, air flow speed is 1000m / s, spraying hard corundum particles with particle size of 1.5μm, the surface roughness Ra=0.9μm after treatment. The TEM image of the surface nanocrystallization by supersonic particle bombardment technology is shown in the attached Figure 1 , from which the surface state after surface nanocrystallization can be seen; the thermal fatigue test surface crack comparison chart of the non-surface nanocrystallization (left) and surface nanocrystallization (right) after 6000 cycles is shown in the attached Figure 2 , from which it can be seen that the thermal fatigue crack of the surface nanocrystallization is far lower than that of the non-surface nanocrystallization.

[0043] Step S7: surface plasma boronizing, boronizing temperature is 775℃ for 4h, boronizing layer thickness is 0.075mm. The SEM image of the wear surface morphology of the sample after plasma boronizing treatment (left) and without plasma boronizing treatment (right) under a load of 45N is shown in the attached Figure 3 , from which it can be seen that the wear of the sample after plasma boronizing treatment is far lower than that of the sample without plasma boronizing treatment.

[0044] Example 2

[0045] Chemical composition (mass percent): C: 0.28%, La: 0.02%, Ce: 0.02%, Mn: 0.4%, Si: 0.1%, Cr: 4.5%, W: 1.5%, Mo: 2.2%, V: 0.8%, Co: 1.0%, Nb: 0.1%, the balance of Fe and impurities, and the contents of S and P are both ≤0.015%.

[0046] Preparation method:

[0047] Step S1: pouring by using a capped mouth and adding a protective slag, and demolding after pouring for 3 h. The electrode rod is annealed by heating at a gradient of 100 ℃ / h to 900 ℃ and keeping for 7 h, and then cooled to 300 ℃ at a cooling rate of 50 ℃ / h. The electrode rod cap is cut off before secondary current remelting, and the oxide skin on the surface of the electrode rod is removed, and heated to 880 ℃ at a heating rate of 100 ℃ / h and kept for 6 h, and then cooled to 300 ℃ at a cooling rate of 40 ℃ / h and discharged from the furnace. The electroslag remelting adopts a CaF2-Al2O3-CaO slag system, the current density is 5 A / cm², the remelting rate is 3 kg / min, and the purity is improved to ≤0.5 level of inclusions.

[0048] Step S2: high-temperature homogenization is heated to 1250 ℃ at a heating rate of 90 ℃ / h and kept for 6 h. A layer of mixed nano metal powder is spread on the surface, hydrogen flame is burned for 5 min, and multi-directional forging is started at a forging temperature of 1150 ℃, the reheating temperature is 1250 ℃, the forging ratio is ≥6, and the final forging temperature is ≥900 ℃.

[0049] Step S3: cold bending deformation amount is 15%, spheroidizing annealing is started at 800 ℃ for 4 h, and then air-cooled after furnace cooling to 500 ℃; deep cooling is liquid nitrogen treatment at -196 ℃ for 2 h.

[0050] Step S4: step quenching is austenitized at 1020 ℃, kept for 1 h, and then oil-cooled to 200 ℃; primary tempering is maintained at 560 ℃ for 3 h, and then air-cooled; secondary tempering is maintained at 520 ℃ for 2 h, and then air-cooled.

[0051] Step S5: 2M hydrochloric acid ethanol solution is sprayed and corroded for 2 min, washed with pure water, multi-element co-diffusion temperature is 520 ℃, co-diffusion time is 6 h, co-diffusion layer thickness is 0.15 mm, and surface hardness is ≥1100 HV.

[0052] Step S6: surface nanocrystallization is performed by supersonic particle bombardment technology, 10 MPa high-pressure compressed air is used, air flow speed is 700 m / s, hard corundum particles with a particle size of 1 μm are sprayed, and the surface roughness Ra of the treated surface is 0.8 μm.

[0053] Step S7: surface plasma boronizing is performed at a boronizing temperature of 750 ℃ for 3 h, and the boronizing layer thickness is 0.05 mm.

[0054] Example 3

[0055] Chemical composition (mass percent): C: 0.35%, La: 0.01%, Ce: 0.01%, Mn: 0.45%, Si: 0.2%, Cr: 5.5%, W: 2.0%, Mo: 2.8%, V: 1.2%, Co: 1.5%, Nb: 0.3%, the balance of Fe and impurities, and the contents of S and P are both ≤0.015%.

[0056] Preparation method:

[0057] Step S1: pouring by using a capped mouth and adding a protective slag, and demolding after pouring for 3h. The electrode rod is annealed by heating at a gradient of 100℃ / h to 900℃ and keeping for 7h, and then furnace cooling to 300℃ at a cooling speed of 50℃ / h. The electrode rod cap is cut off before secondary current remelting, the oxide skin on the surface of the electrode rod is removed, and heating is carried out to 880℃ at a heating speed of 100℃ / h and keeping for 6h, and then furnace cooling to 300℃ at a cooling speed of 40℃ / h and discharging. The electroslag remelting adopts CaF2-Al2O3-CaO slag system, the current density is 8A / cm², the remelting rate is 5kg / min, and the purity is improved to the level of ≤0.5% of inclusions.

[0058] Step S2: high-temperature homogenization is heated to 1250℃ at a heating speed of 90℃ / h and keeping for 8h. A layer of mixed nano metal powder is spread on the surface, hydrogen flame is burned for 5min, and multi-directional forging is started at a forging temperature of 1180℃, the reheating temperature is 1250℃, the forging ratio is ≥6, and the final forging temperature is ≥900℃.

[0059] Step S3: cold bending deformation amount is 20%, spheroidizing annealing is started at 800℃ for 4h, and then air cooling after furnace cooling to 500℃; deep cooling is liquid nitrogen treatment at -196℃ for 2h.

[0060] Step S4: step quenching is austenitizing at 1050℃, keeping for 1h, and then oil cooling to 200℃; primary tempering is maintained at 560℃ for 3h, and then air cooling; secondary tempering is maintained at 520℃ for 2h, and then air cooling.

[0061] Step S5: 2M hydrochloric acid ethanol solution is sprayed for 2min, and then cleaned by purified water, multi-element co-diffusion temperature is 520℃ for 6h, co-diffusion layer thickness is 0.20mm, and surface hardness is ≥1100HV.

[0062] Step S6: surface nanocrystallization is carried out by supersonic particle bombardment technology, 15MPa high-pressure compressed air is used, air flow speed is 1200m / s, hard corundum particles with a particle size of 2μm are sprayed, and the surface roughness after treatment is Ra=1μm.

[0063] Step S7: surface plasma boronizing is carried out at a boronizing temperature of 800℃ for 5h, and the boronizing layer thickness is 0.10mm.

[0064] Comparative Example 1

[0065] La, Ce free, the rest same as Example 1.

[0066] Preparation method: same as Example 1.

[0067] Comparative Example 2

[0068] Ingredients same as Example 1.

[0069] Preparation method:

[0070] Steps S1 to S4 same as Example 1.

[0071] Skip step S5.

[0072] Steps S6 and S7 same as Example 1.

[0073] Comparative Example 3

[0074] Ingredients same as Example 1.

[0075] Preparation process:

[0076] Omitting the surface spreading of a layer of mixed nano metal powder, hydrogen flame burning for 5 min.

[0077] Comparative Example 4

[0078] Ingredients same as Example 1.

[0079] Preparation process:

[0080] Omitting steps S6 and S7, only carrying out multi-element co-permeation.

[0081] Table 1 Test results of examples and comparative examples

[0082]

[0083] From the data of Comparative Example 1 and Example 1, it can be seen that after removing La and Ce, the hardness decreases by 4 HRC, the surface hardness decreases by 170 HV, the impact toughness decreases by 10 J / cm2, and the thermal fatigue life is significantly shortened. It can be concluded that rare earth elements refine the grain size, purify the grain boundary, and improve the strength and toughness and the thermal fatigue resistance of the material. From the data of Comparative Example 3 and Example 1, it can be seen that after omitting the nano powder, the surface hardness decreases by 350 HV, the impact toughness decreases by 13 J / cm2, and the thermal fatigue life decreases by 2000 times. It can be concluded that nano metal powder significantly improves the surface hardness and crack propagation resistance through surface alloying and grain refinement. From the data of Comparative Example 2 and Example 1, it can be seen that after skipping the multi-element co-diffusion, the surface hardness decreases by 100 HV, and the thermal fatigue life decreases by 1000 times. It can be concluded that multi-element co-diffusion forms a high-hardness diffusion layer (≥1100 HV) to enhance the surface wear resistance and thermal fatigue resistance. From the data of Comparative Example 4 and Example 1, it can be seen that after omitting S6 and S7, the surface hardness decreases by 250 HV, and the thermal fatigue life decreases by 2580 times. It can be seen that the gradient nano structure formed by surface nanocrystallization improves the bonding strength of the boronizing layer. From the examples and comparative examples, it can be seen that the examples significantly improve the high-temperature hardness, thermal fatigue resistance, and wear resistance of the die steel through composition optimization: rare earth strengthening grain boundary, high Mo / W / Co improving thermal stability, and process innovation deep cryogenic treatment and composite surface modification. The comparative examples verify the necessity of the key parameters in the patent technology. Deviation of composition or process will result in a significant decrease in performance.

[0084] The above-described examples only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the present patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are within the scope of the present application. Therefore, the protection scope of the present patent should be subject to the appended claims.

Claims

1. A large-scale integrated die-casting special hot work die steel, characterized in that, The chemical composition of the molten steel, by mass percentage, includes: C: 0.28-0.35%, La: 0.01-0.025%, Ce: 0.01-0.02%, Mn: 0.4-0.45%, Si: 0.1-0.2%, Cr: 4.5-5.5%, W: 1.50-2.00%, Mo: 2.2-2.8%, V: 0.8-1.2%, Co: 1.0-1.5%, Nb: 0.1-0.3%, with the balance being Fe and impurities, and the contents of S and P are both ≤0.015%. The raw materials were vacuum-melted in proportion using vacuum induction melting and electroslag remelting methods. A protective slag was added before casting, and the casting was followed by cold annealing with release sand to form electrode rods. The electrode rods were then annealed and subjected to a second electroslag remelting process to obtain steel ingot 1. Specifically, the electrode rods were annealed by gradient heating to 900℃ at a heating rate of 100℃ / h and held for 7 hours, followed by furnace cooling to 300℃ at a cooling rate of 50℃ / h. Before the second current remelting, the electrode rod caps were removed, and the oxide scale on the electrode rod surface was cleaned. The rods were then heated to 880℃ at a heating rate of 100℃ / h and held for 6 hours, followed by cooling at a cooling rate of 40℃ / h. The furnace was cooled to 300℃ and then removed from the furnace. Electroslag remelting used a CaF2-Al2O3-CaO slag system. The resulting steel ingot 1 was homogenized at high temperature, and a layer of mixed nano-metal powder was spread on the surface. After being ignited in a hydrogen flame for 5 minutes, it underwent three upsetting and three drawing multi-directional forging to obtain steel ingot 2. The high-temperature homogenization involved heating to 1250℃ at a rate of 90℃ / h and holding for 6-8 hours. The mixed nano-metal powder consisted of titanium:silver:nickel:iron = 1:1:2:20 (by mass). The initial forging temperature for multi-directional forging was 1150-1180℃. The furnace heating temperature is 1250℃, the forging ratio is ≥6, and the final forging temperature is ≥900℃. The resulting steel ingot 2 is immediately subjected to cold deformation-spheroidizing annealing and deep cryogenic treatment after forging to obtain steel ingot 3. The cold deformation is cold bending deformation with a deformation amount of 10%-20%. The resulting steel ingot 3 undergoes graded quenching and two tempering treatments to obtain steel ingot 4. The graded quenching involves austenitizing at 1020-1050℃, holding for 1 hour, and then oil cooling to 200℃. The first tempering is held at 560℃ for 3 hours, followed by air cooling. The second tempering is held at 520℃ for 2 hours, followed by air cooling. The resulting steel ingot...

4. After surface etching, multi-element co-diffusion is performed to obtain steel ingot 5. The surface etching is carried out by spray etching with 2M hydrochloric acid ethanol solution for 2 min, followed by rinsing with purified water. The multi-element co-diffusion temperature is 520℃ for 6 h. The infiltrator is a mixed solution of tetramethylthiourea:ammonium borate:propionamide:ethanol = 1:0.5:6:10, with the ratio being by mass. The co-diffusion layer thickness is 0.15-0.20 mm, and the surface hardness is ≥1100 HV. The obtained steel ingot 5 is then subjected to surface nano-sizing using supersonic particle bombardment technology to obtain steel ingot 6. The obtained steel ingot 6 is then subjected to surface plasma boronizing to obtain the finished product.

2. A method for preparing large-scale integrated die-casting special hot work die steel as described in claim 1, characterized in that, Includes the following steps: S1: The raw materials are vacuum melted in proportion by vacuum induction melting and electroslag remelting, and protective slag is added for casting. The casting release sand is cold annealed to form an electrode rod. The electrode rod is annealed and subjected to secondary electroslag remelting. The annealed steel ingot 1 is obtained. S2: After homogenizing the steel ingot 1 obtained in step S1 at high temperature, a layer of mixed nano metal powder is spread on the surface. After being calcined with hydrogen flame for 5 minutes, it is subjected to three upsetting and three drawing multi-directional forging to obtain steel ingot 2. S3: The steel ingot 2 obtained in step S2 is immediately subjected to cold deformation-spheroidizing annealing and deep cryogenic treatment after forging to obtain steel ingot 3; S4: The steel ingot 3 obtained in step S3 is subjected to graded quenching and two tempering treatments to obtain steel ingot 4. S5: After etching the surface of the steel ingot 4 obtained in step S4, perform multi-element co-diffusion to obtain steel ingot 5; S6: The steel ingot 5 obtained in step S5 is subjected to surface nano-sizing using supersonic particle bombardment technology to obtain steel ingot 6. S7: Perform surface plasma boronizing on the steel ingot 6 obtained in step S6 to obtain the finished product.

3. The method for preparing large-scale integrated die-casting special hot work die steel according to claim 2, characterized in that, In step S1, the pouring is carried out using the capped pouring method with protective slag added. Demolding is performed 3 hours after pouring, with a current density of 5-8 A / cm³. 2 The remelting rate is 3-5 kg / min, which improves the purity to an inclusion level of ≤0.

5.

4. The method for preparing large-scale integrated die-casting special hot work die steel according to claim 2, characterized in that, In step S3, after deformation, the material is heated and kept at 800°C. The spheroidizing annealing is carried out at 800°C for 4 hours, followed by furnace cooling to 500°C and then air cooling. The cryogenic treatment is carried out at -196°C with liquid nitrogen for 2 hours.

5. The method for preparing large-scale integrated die-casting special hot work die steel according to claim 2, characterized in that, In step S6, the surface nano-sizing method of supersonic particle bombardment technology is as follows: compressed air is used as the carrier gas. The compressed air obtains supersonic speed through the Lava nozzle and carries hard corundum particles with a particle size of 1-2 μm. The particles are sprayed onto the surface of a material that has been ground to a roughness of Ra=0.8-1 μm by a grinding machine. This causes strong plastic deformation of the material surface layer, resulting in finer surface grains and gradually achieving nano-sizing.

6. The method for preparing large-scale integrated die-casting special hot work die steel according to claim 5, characterized in that, The device for nano-sizing of metal surfaces by supersonic particle bombardment consists of four parts: a supersonic spray gun and operating manipulator, a stage, and a parameter monitoring and automatic control system. The supersonic spray gun is designed based on the Laval nozzle principle. The airflow enters the narrow throat through the contraction section and then the expansion section to achieve a supersonic speed of 700~1200 m / s. The air source is 10-15 MPa high-pressure compressed air, which is divided into two paths: one path leads to the powder feeder, carrying the powder into the spray gun; the other path connects to the heater and then enters the spray gun. The spray gun is fixed on the manipulator, enabling three-dimensional, six-directional control. The workpiece is placed on the stage in the operating chamber, which can rotate 360°. The operating chamber is connected to the powder recovery system, and the recovered powder can be reused. It is necessary to check whether each fixing bolt is tightened. After processing, the sample is taken out after cooling to room temperature and then cleaned with alcohol to remove some residual debris and iron filings, finally obtaining a nano-sized surface. It is then placed in a dry environment to prevent surface oxidation.

7. The method for preparing large-scale integrated die-casting special hot work die steel according to claim 2, characterized in that, In step S7, the surface plasma boronizing method is as follows: the surface of the nano-sized sample is cleaned with cyclohexane and dried, then placed in a cleaned plasma heat treatment furnace. The sealed area is wiped clean with cyclohexane, the furnace lid is closed, and the vacuum pump is turned on. The system is evacuated to below 0.01 kPa, and the current and voltage are turned on to enter the arc ignition stage. After the arc ignition is completed, gas is introduced, first a mixture of argon and hydrogen in a ratio of 2-4:1, to enter the heating stage. At the beginning of the heating stage, the current is increased to improve the heating efficiency. As the temperature increases, the current gradually decreases to prevent overheating. Once the temperature reaches 750-800℃ and stabilizes, 1-2% of the total volume of argon and hydrogen in diborane is introduced and the mixture is kept at this temperature for 3-5 hours. After the holding period, the current and voltage are turned off, and the residual gas in the furnace and pipes is removed. Then, a certain amount of argon is introduced to allow the sample to cool to room temperature in the protective atmosphere of argon. The surface plasma boronizing temperature is 750-800℃ for 3-5 hours, and the boronized layer thickness is 0.05-0.10 mm.

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