Special hot work die steel for large-scale integrated die-casting process and preparation method of special hot work die steel

Abstract

The invention discloses special hot work die steel for large-scale integrated die casting and a preparation method of the special hot work die steel, and belongs to the technical field of hot work die steel. The steel comprises the following chemical components in percentage by mass: 0.28%-0.35% of C, 0.01%-0.025% of La, 0.01%-0.02% of Ce, 0.4%-0.45% of Mn, 0.1%-0.2% of Si, 4.5%-5.5% of Cr, 1.50%-2.00% of W, 2.2%-2.8% of Mo, 0.8%-1.2% of V, 1.0%-1.5% of Co, 0.1%-0.3% of Nb and the balance of Fe and impurities (S and P are less than or equal to 0.015%). The preparation method comprises the composite processes of vacuum induction melting and electroslag remelting purification, hydrogen firing high-temperature homogenization and three-upsetting and three-drawing multi-directional forging, spheroidizing annealing and subzero treatment, step quenching and double tempering, multi-element co-permeation, supersonic particle bombardment surface nanocrystallization and plasma boriding. The rare earth element La / Ce refined grain boundary is combined with multi-element alloy strengthening and surface nanocrystallization-boronizing synergistic treatment, so that the hardness, impact toughness and thermal fatigue life of the die steel are remarkably improved, and the die steel is suitable for high-temperature, high-pressure and high-wear working conditions of a large integrated die-casting die.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and in particular to a hot working die steel specially used for a large integrated die casting process and a preparation method thereof, in particular to a die steel material which serves for a long time under high pressure and high temperature conditions. Background Art

[0002] With the rapid development of new energy vehicles and lightweight structural parts, large-scale integrated die-casting technology has been widely used due to its advantages such as high efficiency and high precision. However, traditional hot working die steel (such as H13, 8407) is prone to thermal fatigue cracks, high temperature softening, oxidation wear and other problems when subjected to high temperature and high pressure molten metal impact above 600℃ for a long time, resulting in short die life and high maintenance cost.

[0003] In the existing technology, high-temperature strength is improved by adding elements such as molybdenum (Mo) and vanadium (V), or purity is improved by electroslag remelting, but the following defects still exist: grain boundary weakening at high temperature and insufficient thermal fatigue resistance; low thermal conductivity leads to local overheating of the mold; and the size effect of large molds leads to uneven structure.

[0004] Therefore, there is an urgent need to develop a new type of hot working die steel that has high thermal conductivity, thermal fatigue resistance, high temperature strength and excellent processing performance. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a hot working die steel specially used for large-scale integrated die-casting process and its preparation method, so as to solve the problems that the current hot working die steel is prone to thermal fatigue cracks, high temperature softening, oxidation wear and other problems when subjected to high temperature and high pressure molten metal impact above 600°C for a long time.

[0006] Technical solution: A large-scale integrated die-casting special hot working die steel, the chemical components of the molten steel include, 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 remainder is Fe and impurities, and the S and P contents are both ≤0.015%.

[0007] A method for preparing a large-scale integrated die-casting special hot working die steel comprises the following steps: S1: vacuum melting the raw materials in proportion by vacuum induction melting and electroslag remelting, adding protective slag for casting, pouring demoulding sand for cold annealing into electrode rods, annealing the electrode rods and performing secondary electroslag remelting, and annealing to obtain steel ingot 1; S2: After subjecting the ingot 1 obtained in step S1 to high-temperature homogenization, a layer of mixed nano-metal powder is spread on the surface, and after being burned with a hydrogen flame for 5 min, it is subjected to three-upsetting and three-drawing multi-directional forging to obtain ingot 2; S3: Immediately after forging the ingot 2 obtained in step S2, cold deformation-spheroidizing annealing and cryogenic treatment are carried out to obtain ingot 3; S4: The ingot 3 obtained in step S3 is subjected to step quenching and two tempering treatments to obtain ingot 4; S5: After surface corrosion of the ingot 4 obtained in step S4, multi-element co-permeation is carried out to obtain ingot 5; S6: The surface of the ingot 5 obtained in step S5 is subjected to surface nano-crystallization by supersonic particle bombardment technology to obtain ingot 6; S7: Surface plasma boronizing is carried out on the ingot 6 obtained in step S6 to obtain the finished product.

[0008] Preferably, in step S1, the casting is carried out by the bottom-pouring method with a riser and protective slag is added for casting. After casting for 3 h, demolding is carried out. The electrode bar annealing is to heat it to 900 °C at a heating rate of 100 °C / h and hold for 7 h, and then cool it in the furnace to 300 °C, with a cooling rate of 50 °C / h. Before secondary current remelting, the riser of the electrode bar is cut off, the oxide scale on the surface of the electrode bar is removed, it is heated to 880 °C at a heating rate of 100 °C / h and held for 6 h, and then cooled in the furnace to 300 °C at a cooling rate of 40 °C / h and taken out of the furnace. The electroslag remelting uses a CaF2-Al2O3-CaO slag system, with a current density of 5-8 A / cm² and a remelting rate of 3-5 kg / min, and the purity is improved to inclusions ≤ 0.5 grade.

[0009] Preferably, in step S2, the high-temperature homogenization is to heat it to 1250 °C at a heating rate of 90 °C / h and hold for 6-8 h. The mixed nano-metal powder is a mixed nano-metal powder of titanium: silver: nickel: iron (w / w) = 1:1:2:20. The starting forging temperature of the multi-directional forging is 1150-1180 °C, the reheating temperature is 1250 °C, the forging ratio ≥ 6, and the final forging temperature ≥ 900 °C.

[0010] Preferably, in step S3, the cold deformation is cold bending deformation, with a deformation amount of 10%-20%. After deformation, heating and holding are carried out, heating to 800 °C. The spheroidizing annealing is to start annealing at 800 °C for 4 h, and then cool it in the furnace to 500 °C and then air-cool. The cryogenic treatment is to treat it with liquid nitrogen at -196 °C for 2 h.

[0011] Preferably, in step S4, the step quenching is to carry out austenitization at 1020-1050 °C, hold for 1 h and then oil-cool to 200 °C; the first tempering is to maintain at 560 °C for 3 h and then air-cool; the second tempering is to maintain at 520 °C for 2 h and then air-cool.

[0012] Preferably, in step S5, the surface corrosion is carried out by spraying and corroding with 2M hydrochloric acid ethanol solution for 2 min, followed by rinsing thoroughly with purified water. The multi-element co-permeation temperature is 520 °C for 6 h, and the co-permeation agent is a mixed solution of tetramethylthiourea: ammonium borate: propionamide: ethanol (W / W) = 1:0.5:6:10. The thickness of the co-permeation layer is 0.15 - 0.20 mm, and the surface hardness is ≥1100 HV.

[0013] Preferably, in step S6, the method for surface nanocrystallization by supersonic particle bombardment technology is as follows: Compressed air is used as the carrier gas. The compressed air obtains supersonic speed through a Laval nozzle and carries corundum, a hard particle with a particle size of 1 - 2 μm, and sprays it onto the surface of a material that has been ground to a roughness Ra = 0.8 - 1 μm by a grinding machine. Due to the countless high-speed repeated impacts of the particles on the surface of the material, the surface layer of the material undergoes severe plastic deformation, resulting in the refinement of surface grains and gradually achieving nanocrystallization. The supersonic particle bombardment device for metal surface nanocrystallization consists of four parts: a supersonic spray gun and an operating manipulator, a loading table, 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 air flow enters the narrow throat through the contraction section and then enters the expansion section to obtain a supersonic speed of 700 - 1200 m / s. The gas source uses high-pressure compressed air of 10 - 15 MPa. The gas is divided into two paths: one path leads to the powder feeder, carrying the powder into the spray gun; the other path is connected to the heater and then enters the spray gun. The spray gun can be fixed on the manipulator to achieve three-dimensional six-direction control. The workpiece is placed on the loading table in the operating chamber. The loading table can rotate 360°. The operating chamber is connected to a powder recovery system. After the powder is recovered, it can be reused. Then, it is necessary to check whether each fixing bolt is tightened. After the treatment is completed, the sample is taken out after cooling to room temperature. The surface is cleaned again with alcohol to wash away some garbage and iron filings remaining on the surface. Finally, a nanocrystallized surface is obtained and placed in a dry environment to prevent surface oxidation.

[0014] Preferably, in step S7, the surface plasma boronizing method is as follows: Clean and dry the surface of the surface-nanocrystallized specimen with cyclohexane, place it in a plasma heat treatment furnace that has been wiped clean, wipe the sealing part with cyclohexane, cover the furnace lid and then turn on the vacuum pump. After the system is pumped down to below 0.01 KPa, turn on the current and voltage to enter the arc striking stage. After the arc striking is completed, start to introduce gas. First, introduce a mixed gas of argon:hydrogen = 2 - 4:1, and start to enter the heating stage. At the beginning stage, the current can be adjusted to be larger to improve the heating efficiency. As the temperature rises, the current should be gradually reduced to prevent runaway temperature. After the temperature rises to 750 - 800 °C and stabilizes, introduce diborane accounting for 1 - 2% of the total volume of argon and hydrogen, and keep the temperature for 3 - 5 h. After the heat preservation ends, turn off the current and voltage, evacuate the residual gas in the furnace body and pipeline, and then introduce a certain amount of argon to cool the specimen to room temperature in the protective atmosphere of argon. The surface plasma boronizing temperature is 750 - 800 °C for 3 - 5 h, and the thickness of the boronizing layer is 0.05 - 0.10 mm.

[0015] Beneficial effects:

[0016] 1. By means of vacuum induction melting and electroslag remelting methods, and using the 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, thus significantly increasing the service life of the mold.

[0017] 2. Cold deformation - spheroidizing annealing is adopted to make the carbides in the die steel distribute in a spherical shape, reducing the splitting effect of the carbides on the matrix, improving the toughness and machining performance of the material. Cryogenic treatment further refines the grains and enhances the strength and hardness of the die steel.

[0018] 3. By high - speed particle impact on the surface of the die steel, surface nanocrystallization is achieved, making the surface grains refined to the nanoscale. This nanocrystallized surface not only improves the wear resistance of the mold but also effectively reduces surface wear and the generation of fatigue cracks during the use of the mold.

[0019] 4. A multi - element infiltration layer with a thickness of 0.15 - 0.20 mm is formed on the surface of the die steel, and the surface hardness reaches ≥1100 HV. The multi - element infiltration layer significantly improves the wear resistance and anti - galling performance of the mold, reduces the adhesion phenomenon between the mold and the die - casting material, and thus extends the service life of the mold.

[0020] 5. On the basis of the nanocrystallized surface, plasma boronizing treatment is carried out to form a boronizing layer with a thickness of 0.05 - 0.10 mm. The boronizing layer further improves the surface hardness and wear resistance of the mold, and at the same time enhances the corrosion resistance of the mold, enabling it to maintain good performance in high - temperature and corrosive environments.

[0021] 6. The mixed nano-metal powder can strengthen the surface, improve hardness and wear resistance; the nano-particles pin the grain boundaries, and combined with multi-directional forging, the grains are refined; silver reduces forging defects and improves formability; it provides a reaction source for subsequent boronizing and co-cementation, enhancing the performance of the cemented layer. Brief Description of the Drawings

[0022] Figure 1 This is the surface TEM image of the present invention after surface nanocrystallization by the supersonic particle bombardment technology.

[0023] Figure 2 This is a comparison diagram of the surface cracks in the thermal fatigue test of 6000 cycles without surface nanocrystallization (left) and with surface nanocrystallization (right).

[0024] Figure 3 This is the SEM image of the worn surface morphology of the specimen after plasma boronizing treatment (left) and without plasma boronizing treatment (right) under a load of 45 N.

[0025] Figure 4 This is the X-ray diffraction pattern of surface multi-element co-cementation and without co-cementation. Detailed Description of the Embodiments

[0026] To make the technical solution of the present invention clearer, the following further describes the present invention in detail with reference to the accompanying drawings and specific embodiments.

[0027] Example 1

[0028] Chemical composition (mass percentage): 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 is Fe and impurities, and the contents of S and P are both ≤ 0.015%.

[0029] Preparation method: Step S1: Pouring is carried out by the bottom-pouring method with a capping and adding flux. After pouring for 3 h, demolding is carried out. The electrode bar is annealed by heating at a gradient of 100 °C / h to 900 °C and holding for 7 h, and then cooled in the furnace to 300 °C, and the cooling rate is 50 °C / h. Before secondary current remelting, the capping of the electrode bar is removed, the oxide scale on the surface of the electrode bar is removed, heated at a heating rate of 100 °C / h to 880 °C and held for 6 h, and then cooled in the furnace to 300 °C at a cooling rate of 40 °C / h and taken out of the furnace. Electroslag remelting uses a CaF2 - Al2O3 - CaO slag system, the current density is 6 A / cm², the remelting rate is 4 kg / min, and the purity is improved to inclusions ≤ grade 0.5.

[0030] Step S2: High-temperature homogenization: Heat at a heating rate of 90 °C / h to 1250 °C and hold for 7 h. Spread a layer of mixed nano-metal powder on the surface, burn it with a hydrogen flame for 5 min. The initial forging temperature for multi-directional forging is 1160 °C, the reheating temperature for furnace return is 1250 °C, the forging ratio ≥ 6, and the final forging temperature ≥ 900 °C.

[0031] Step S3: Cold bending deformation with a deformation amount of 10%. Start annealing at 800 °C for 4 h and then cool in the furnace to 500 °C and then air-cool. Deep cooling is carried out with liquid nitrogen at -196 °C for 2 h.

[0032] Step S4: Step quenching: Austenitize at 1030 °C, hold for 1 h and then oil-cool to 200 °C; temper once at 560 °C for 3 h and then air-cool; temper twice at 520 °C for 2 h and then air-cool.

[0033] Step S5: Spray corrosion with 2M hydrochloric acid ethanol solution for 2 min, rinse thoroughly with purified water. Carry out multi-element co-permeation at 520 °C for 6 h. The thickness of the co-permeation layer is 0.18 mm, and the surface hardness ≥ 1100 HV. Among them, the X-ray diffraction patterns of the surface multi-element co-permeation and the non-co-permeated ones are shown in the appendix Figure 4 , It can be seen from the attached figure that compared with the non-surface multi-element co-permeation, the X-ray diffraction peaks on the surface of the specimen after surface multi-element co-permeation have obvious broadening.

[0034] Step S6: Carry out surface nanocrystallization by supersonic particle bombardment technology. Use high-pressure compressed air at 12 MPa, the air flow velocity is 1000 m / s, and spray corundum with a particle size of 1.5 μm as the hard particle. The surface roughness Ra = 0.9 μm after treatment. Among them, the surface TEM image after surface nanocrystallization by supersonic particle bombardment technology is shown in the appendix Figure 1 , The surface state after surface nanocrystallization can be seen from the attached figure; The comparison diagram of the surface cracks of the thermal fatigue test of non-surface nanocrystallization (left) and surface nanocrystallization (right) after 6000 cycles is shown in the appendix Figure 2 , It can be seen from the attached figure that the thermal fatigue cracks of surface nanocrystallization are much lower than those of non-surface nanocrystallization.

[0035] Step S7: Surface plasma boronizing, boronizing temperature is 775 °C for 4 h, and the thickness of the boronizing layer is 0.075 mm. Among them, the SEM images of the worn surface morphology of the specimens after plasma boronizing treatment (left) and without plasma boronizing treatment (right) under a load of 45 N are shown in the appendix Figure 3 , It can be seen from the attached figure that the wear after plasma boronizing treatment is much lower than the surface wear without plasma boronizing treatment.

[0036] Example 2

[0037] Chemical composition (mass percentage): 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 is Fe and impurities, and the contents of S and P are both ≤ 0.015%.

[0038] Preparation method: Step S1: Pour using the bottom-pouring method with a capping and add protective slag. Demold after casting for 3 h. Anneal the electrode bar by heating it at a gradient of 100 °C / h to 900 °C and holding for 7 h, then cool it in the furnace to 300 °C, and the cooling rate is 50 °C / h. Before secondary current remelting, cut off the capping of the electrode bar, remove the oxide scale on the surface of the electrode bar, heat it at a heating rate of 100 °C / h to 880 °C and hold for 6 h, then cool it in the furnace at a cooling rate of 40 °C / h to 300 °C and take it out of the furnace. For electroslag remelting, use the 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 inclusions ≤ 0.5 grade.

[0039] Step S2: High-temperature homogenization: Heat at a heating rate of 90 °C / h to 1250 °C and hold for 6 h. Spread a layer of mixed nano-metal powder on the surface, burn it with a hydrogen flame for 5 min. The starting forging temperature for multi-directional forging is 1150 °C, the reheating temperature for returning to the furnace is 1250 °C, the forging ratio ≥ 6, and the final forging temperature ≥ 900 °C.

[0040] Step S3: Cold bending deformation: The deformation amount is 15%. Spheroidizing annealing starts at 800 °C and anneals for 4 h, then cools in the furnace to 500 °C and then air cools; Deep cooling is liquid nitrogen treatment at -196 °C for 2 h.

[0041] Step S4: Step quenching: Austenitize at 1020 °C, hold for 1 h and then oil cool to 200 °C; First temper at 560 °C for 3 h, then air cool; Second temper at 520 °C for 2 h, then air cool.

[0042] Step S5: Spray corrosion with 2M hydrochloric acid ethanol solution for 2 min, rinse with purified water. Multielement co-permeation: Co-permeate at 520 °C for 6 h, the co-permeation layer thickness is 0.15 mm, and the surface hardness ≥ 1100 HV.

[0043] Step S6: Perform surface nanocrystallization by supersonic particle bombardment technology. Use 10 MPa high-pressure compressed air, the air flow velocity is 700 m / s, spray hard particles corundum with a particle size of 1 μm, and the surface roughness Ra after treatment is 0.8 μm.

[0044] Step S7: Surface plasma boronizing, boronizing temperature 750 °C, boronizing time 3 h, boronizing layer thickness 0.05 mm.

[0045] Example 3

[0046] Chemical composition (mass percentage): 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 is Fe and impurities, and the contents of S and P are both ≤ 0.015%.

[0047] Preparation method: Step S1: Pouring is carried out by the bottom-pouring method with a riser and adding flux. After pouring for 3 h, demoulding is carried out. The electrode bar is annealed by heating to 900 °C at a gradient of 100 °C / h and holding for 7 h, and then cooled in the furnace to 300 °C with a cooling rate of 50 °C / h. Before secondary current remelting, the riser of the electrode bar is cut off, the oxide scale on the surface of the electrode bar is removed, heated to 880 °C at a heating rate of 100 °C / h and held for 6 h, and then cooled in the furnace to 300 °C with a cooling rate of 40 °C / h and taken out of the furnace. Electroslag remelting uses a CaF2-Al2O3-CaO slag system, with a current density of 8 A / cm², a remelting rate of 5 kg / min, and the purity is improved to inclusions ≤ 0.5 grade.

[0048] Step S2: High-temperature homogenization is carried out by heating to 1250 °C at a heating rate of 90 °C / h and holding for 8 h. A layer of mixed nano-metal powder is spread on the surface, burned with a hydrogen flame for 5 min. The multi-directional forging starting forging temperature is 1180 °C, the reheating temperature for returning to the furnace is 1250 °C, the forging ratio ≥ 6, and the final forging temperature ≥ 900 °C.

[0049] Step S3: Cold bending deformation with a deformation amount of 20%, spheroidizing annealing starts annealing at 800 °C for 4 h, furnace cooling to 500 °C and then air cooling; cryogenic treatment is -196 °C liquid nitrogen treatment for 2 h.

[0050] Step S4: Step quenching is carried out for austenitization at 1050 °C, oil cooling to 200 °C after holding for 1 h; the first tempering is maintained at 560 °C for 3 h and then air cooling; the second tempering is maintained at 520 °C for 2 h and then air cooling.

[0051] Step S5: Spray corrosion with 2M hydrochloric acid ethanol solution for 2 min, rinse with purified water, multi-element co-permeation at a temperature of 520 °C for 6 h, the co-permeation layer thickness is 0.20 mm, and the surface hardness ≥ 1100 HV.

[0052] Step S6: Surface nanocrystallization is carried out by the supersonic particle bombardment technology, using 15 MPa high-pressure compressed air, the air flow velocity is 1200 m / s, and the hard particles corundum with a jet particle size of 2 μm is sprayed. The surface roughness Ra after treatment is 1 μm.

[0053] Step S7: Surface plasma boronizing, boronizing temperature 800 °C for boronizing for 5 h, boronizing layer thickness 0.10 mm.

[0054] Comparative Example 1

[0055] Without La and Ce, the rest is the same as in Example 1.

[0056] Preparation method: The same as in Example 1.

[0057] Comparative Example 2

[0058] The components are the same as in Example 1.

[0059] Preparation method: Steps S1 to S4 are the same as in Example 1.

[0060] Skip step S5.

[0061] Steps S6 and S7 are the same as in Example 1.

[0062] Comparative Example 3

[0063] The components are the same as in Example 1.

[0064] Preparation process: Omit spreading a layer of mixed nano metal powder on the surface and burn it with a hydrogen flame for 5 minutes.

[0065] Comparative Example 4

[0066] The components are the same as in Example 1.

[0067] Preparation process: Omit steps S6 and S7 and only perform multi - element co - infiltration.

[0068] Table 1 Test results of examples and comparative examples

[0069] From the data of Comparative Example 1 and Example 1, it can be seen that after removing La and Ce, the hardness decreased by 4 HRC, the surface hardness decreased by 170 HV, the impact toughness decreased by 10 J / cm², and the thermal fatigue life was significantly shortened. It can be concluded that rare earth elements improve the strength, toughness and thermal fatigue resistance of the material by refining grains and purifying grain boundaries; from the data of Comparative Example 3 and Example 1, it can be seen that after omitting the nano-powder, the surface hardness dropped sharply by 350 HV, the impact toughness decreased by 13 J / cm², and the thermal fatigue life decreased 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-permeation, the surface hardness decreased by 100 HV and the thermal fatigue life decreased by 1000 times. It can be concluded that multi-element co-permeation forms a high-hardness permeation layer (≥1100 HV), enhancing 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 decreased by 250 HV and the thermal fatigue life decreased by 2580 times. It can be known that the bonding force of the boronized layer is improved by forming a gradient nanostructure through surface nanocrystallization. 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 die steel through composition optimization: rare earth strengthening grain boundaries, high Mo / W / Co improving thermal stability, and process innovation of cryogenic treatment and composite surface modification. The comparative examples verify the necessity of the key parameters in the patented technology, and deviation from the composition or process will lead to a significant decline in performance.

[0070] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but should not be construed as a limitation to the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent shall be subject to the appended claims.

Claims

1. A hot work die steel special for large-scale integrated die casting, characterized in that, The chemical components of the molten steel by mass percentage include: 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 being Fe and impurities, and the contents of S and P are both ≤ 0.015%.

2. A preparation method of a special hot work die steel for large-scale integrated die casting, characterized in that, It includes the following steps: S1: By means of vacuum induction melting and electroslag remelting methods, the raw materials are melted in vacuum according to the proportion, and pouring is carried out with protective slag. After casting, the demolding sand is cold annealed to form an electrode bar. The electrode bar is annealed and then subjected to secondary electroslag remelting, and annealing is carried out to obtain ingot 1; S2: After the ingot 1 obtained in step S1 is subjected to high-temperature homogenization, a layer of mixed nano-metal powder is spread on the surface. After being burned by a hydrogen flame for 5 min, three-upsetting and three-drawing multi-directional forging is carried out to obtain ingot 2; S3: Immediately after forging the ingot 2 obtained in step S2, cold deformation-spheroidizing annealing and cryogenic treatment are carried out to obtain ingot 3; S4: The ingot 3 obtained in step S3 is subjected to step quenching and two tempering treatments to obtain ingot 4; S5: After the surface of the ingot 4 obtained in step S4 is corroded, multi-element co-permeation is carried out to obtain ingot 5; S6: The surface of the ingot 5 obtained in step S5 is subjected to surface nanocrystallization by supersonic particle bombardment technology to obtain ingot 6; S7: Surface plasma boronizing is carried out on the ingot 6 obtained in step S6 to obtain the finished product.

3. The preparation method of the special hot work die steel for large integrated die casting according to claim 2, characterized in that, In step S1, the pouring is carried out by the under-pouring method with a riser and protective slag for pouring. After pouring for 3 h, demolding is carried out. The annealing of the electrode bar is to heat it at a heating rate of 100 °C / h to 900 °C and hold for 7 h, and then cool it in the furnace to 300 °C with a cooling rate of 50 °C / h. Before the secondary current remelting, the riser of the electrode bar is cut off, the oxide scale on the surface of the electrode bar is removed, it is heated to 880 °C at a heating rate of 100 °C / h and held for 6 h, and then cooled in the furnace to 300 °C at a cooling rate of 40 °C / h and taken out of the furnace. The electroslag remelting uses 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 inclusions ≤ 0.5 grade.

4. According to the preparation method of the hot work die steel for large integrated die casting as described in claim 2, in step S2, the high-temperature homogenization is to heat it to 1250 °C at a heating rate of 90 °C / h and hold for 6 - 8 h. The mixed nano-metal powder is a mixed nano-metal powder of titanium: silver: nickel: iron (w / w) = 1:1:2:

20. The starting forging temperature of the multi-directional forging is 1150 - 1180 °C, the reheating temperature for returning to the furnace is 1250 °C, the forging ratio ≥ 6, and the final forging temperature ≥ 900 °C.

5. The preparation method of the hot work die steel special for large integrated die casting according to claim 2, characterized in that, In step S3, the cold deformation is cold bending deformation, the deformation amount is 10% - 20%, after deformation, heating and holding are carried out, heating to 800 °C, the spheroidizing annealing starts annealing at 800 °C for 4 h, and then air-cooled after furnace cooling to 500 °C; the cryogenic treatment is liquid nitrogen treatment at -196 °C for 2 h.

6. The preparation method of the special hot work die steel for large integrated die casting according to claim 2, characterized in that, In step S4, the marquenching is carried out by austenitizing at 1020 - 1050 °C, holding for 1 h and then oil cooling to 200 °C; the first tempering is carried out at 560 °C for 3 h and then air cooling; the second tempering is carried out at 520 °C for 2 h and then air cooling.

7. The preparation method of the special hot work die steel for large integrated die casting according to claim 2, characterized in that, In step S5, the surface corrosion is carried out by spraying with 2M hydrochloric acid ethanol solution for 2 min and then rinsing clean with purified water. The multi - element co - infiltration temperature is 520 °C for 6 h. The infiltrant is a mixed solution of tetramethylthiourea: ammonium borate: propionamide: ethanol (W / W / W / W) = 1:0.5:6:

10. The thickness of the co - infiltration layer is 0.15 - 0.20 mm and the surface hardness is ≥1100 HV.

8. The preparation method of the special hot work die steel for large integrated die casting according to claim 2, characterized in that, In step S6, the method of surface nanocrystallization by supersonic particle bombardment technology is as follows: Compressed air is used as the carrier gas. The compressed air obtains supersonic speed through a Laval nozzle and carries hard corundum particles with a particle size of 1 - 2 μm and sprays them onto the surface of the material ground to a roughness Ra = 0.8 - 1 μm by a grinding machine, causing severe plastic deformation on the surface layer of the material, resulting in the refinement of surface grains and gradually achieving nanocrystallization.

9. The preparation method of the hot work die steel special for large integral die casting according to claim 8, characterized in that, The device for surface nanocrystallization of metals by supersonic particle bombardment consists of four parts: a supersonic spray gun and an operating manipulator, a loading table, and a parameter monitoring and automatic control system. The supersonic spray gun is designed based on the principle of a Laval nozzle. The air flow enters the narrow throat through the contraction section and then enters the expansion section to obtain a supersonic speed of 700 - 1200 m / s. The gas source uses high - pressure compressed air of 10 - 15 MPa. The gas is divided into two paths: one path leads to the powder feeder to carry the powder into the spray gun; the other path is connected to the heater and then enters the spray gun. The spray gun can be fixed on the manipulator to achieve three - dimensional six - azimuth control. The workpiece is placed on the loading table in the operating chamber. The loading table can rotate 360°. The operating chamber is connected to a powder recovery system. After the powder is recovered, it can be reused. Then, it is necessary to check whether each fixing bolt is tightened. After the treatment is completed, the sample is taken out after cooling to room temperature, and the surface is cleaned again with alcohol to wash away some garbage and iron filings remaining on the surface. Finally, a nanocrystallized surface is obtained and placed in a dry environment to prevent surface oxidation.

10. The preparation method of the special hot work die steel for large integral die casting according to claim 2, characterized in that, In step S7, the method of surface plasma boronizing is as follows: The surface of the sample with surface nanocrystallization is cleaned with cyclohexane and dried, then placed in a plasma heat treatment furnace that has been wiped clean. The sealing part is wiped clean with cyclohexane, the furnace lid is covered, and then the vacuum pump is turned on. The system is pumped to below 0.01 KPa, then the current and voltage are turned on to enter the arcing stage. After the arcing is completed, gas is introduced. First, a mixed gas of argon: hydrogen = 2 - 4:1 is introduced to enter the heating stage. The current is increased at the beginning of the heating stage to improve the heating efficiency. As the temperature rises, the current gradually decreases to prevent runaway temperature. After the temperature rises to 750 - 800 °C and stabilizes, diborane accounting for 1 - 2% of the total volume of argon and hydrogen is introduced and held for 3 - 5 h. After the holding is completed, the current and voltage are turned off, and the residual gas in the furnace body and pipeline is evacuated. Subsequently, a certain amount of argon is introduced to cool the sample to room temperature in the protective atmosphere of argon. The surface plasma boronizing temperature is 750 - 800 °C for 3 - 5 h, and the thickness of the boronizing layer is 0.05 - 0.10 mm.

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