Method for producing a steel for die casting molds

Abstract

The application discloses a special steel for improved die-casting mould, which comprises the following components in percentage by mass: C: 0.36%-0.38%; Si: 0.20%-0.30%; Mn: 0.40%-0.50%; P: <=0.010%; S: <=0.001%; Cr: 4.90%-5.10%; Ni: 0.40%-0.50%; Mo: 2.2%-2.5%; V: 0.50%-0.60%; and the rest is Fe. The application also discloses a preparation method of the special steel for improved die-casting mould, which comprises the following steps: S1 smelting, S2 refining, S3 electroslag remelting, S4 forging treatment, S5 forging treatment, S6 superfine treatment and S7 spheroidizing annealing. The application optimizes the content formula combination of chemical elements on the basis of the original hot work die steel 4Cr5MoSiV material, and then prepares the improved die-casting mould steel through the optimized smelting-refining-electroslag remelting-forging treatment-superfine treatment-spheroidizing annealing process, so that the high-temperature strength and toughness of the die-casting mould are met.

Description

Technical Field

[0001] This invention relates to the field of steel alloy technology, and specifically to a method for preparing special steel for die casting molds. Background Technology

[0002] Aluminum, magnesium, zinc, and titanium alloy die castings are widely used in industries and fields such as aviation, aerospace, nuclear power, petrochemicals, transportation, construction machinery, and medical. Currently, there are many and emerging grades of hot work die steel, but for large product parts and complex mold manufacturing, especially lightweight and integrated structural parts and shells for new energy vehicles, users have always faced shortcomings and limitations in material selection.

[0003] Existing technical literature, application number: 201010224086.8, patent title: A high-toughness, high-isotropic ZW851 hot work die steel, discloses a high-toughness, high-isotropic ZW851 hot work die steel, compatible with high-toughness die casting dies and industrial aluminum profile extrusion dies. This die steel has the characteristics of high toughness and high isotropy. Compared with general-purpose H13 and its improved hot work die steel, it can improve die life and reduce die cost. By reducing Si and V content, controlling the appropriate Mo content, and controlling the residual gas and element content, the material toughness is improved while controlling the increase in cost. The banded structure reaches SA1-SA2, SB1-SB2, and the spheroidized structure reaches AS1-AS4. At a hardness of 44-46 HRC, the transverse and longitudinal V-notch impact energy is ≥19J, and the isotropy is ≥0.9. Suitable for long-life die-casting molds of aluminum alloys, magnesium alloys, zinc alloys, etc., and extrusion molds of 7-series high-strength, high-hardness industrial aluminum profiles. It can also be used for hot forging molds with high toughness requirements and reinforced plastic molds with high hardness and high wear resistance requirements.

[0004] Given market conditions and user needs, there is a growing demand for an improved die-casting mold steel that can meet the manufacturing requirements of large or complex die-casting molds, ensuring overall toughness and safety, while also resisting early cracking and erosion to guarantee its service life. Summary of the Invention

[0005] To overcome the above-mentioned defects, the present invention aims to provide a die casting mold steel and its preparation method. Based on the existing hot work die steel 4Cr5MoSiV type material, the main chemical element content formula combination is optimized, and then the die casting mold steel is prepared through a series of smelting-refining-electroslag remelting-forging treatment-ultra-fine treatment-spheroidizing annealing processes.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A special steel for die casting molds, comprising the following components by mass percentage: C: 0.36%-0.38%; Si: 0.20%-0.30%; Mn: 0.40%-0.50%; P: ≦0.010%; S: ≦0.001%; ​​Cr: 4.90%-5.10%; Ni: 0.40%-0.50%; Mo: 2.2%-2.5%; V: 0.50%-0.60%; with the balance being Fe.

[0008] Further preferred, the die-casting mold special steel comprises the following components by mass percentage: C: 0.36%; Si: 0.20%; Mn: 0.40%; P: ≦0.012%; S: ≦0.001%; ​​Cr: 4.90%; Ni: 0.40%; Mo: 2.20%; V: 0.50%; with the remainder being Fe.

[0009] Further preferred, the die-casting mold special steel comprises the following components by mass percentage: C: 0.37%; Si: 0.25%; Mn: 0.45%; P: ≦0.012%; S: ≦0.003%; Cr: 5.00%; Ni: 0.45%; Mo: 2.35%; V: 0.55%; with the remainder being Fe.

[0010] Further preferred, the die-casting mold special steel comprises the following components by mass percentage: C: 0.38%; Si: 0.30%; Mn: 0.50%; P: ≦0.010%; S: ≦0.001%; ​​Cr: 5.10%; Ni: 0.50%; Mo: 2.50%; V: 0.60%; with the remainder being Fe.

[0011] A method for preparing a special steel for die casting molds, the method comprising the steps of: S1 smelting, S2 refining, S3 electroslag remelting, S4 forging treatment, S5 ultrafine treatment, and S6 spheroidizing annealing, specifically including the following steps:

[0012] S1. Smelting: According to the mass percentage of the components contained in the die casting mold steel, various alloy raw materials are put into the electric arc furnace and heated to gradually melt. Vacuum melting is carried out at a temperature of 1650-1720℃ and held for 5-8 hours to make the temperature and composition of the whole furnace melt uniform.

[0013] S2, Refining: S1 is converted to LF ladle refining, VD vacuum degassing is followed by casting, and natural cooling is used to form steel ingots;

[0014] S3, Electroslag Remelting: The steel ingot formed in S2 is heated to a temperature of 1650-1720℃ using an ESR electroslag remelting equipment, then filtered through a slag pool and naturally cooled to generate a new steel ingot.

[0015] S4. Forging process: The ESR steel ingot formed by S3 is heated to 1260-1280℃ and held for 25-30 hours. Then it is transferred to a forging press for three-dimensional surface forging. The final temperature is controlled at 800-850℃. After cooling to 200-300℃, it is transferred to ultrafine processing.

[0016] S5. Ultra-fine treatment: Slowly heat the S4 forging billet to 650℃ in a converter and hold for 2.5 min / mm. Continue to heat to 850℃ and hold for 2.5 min / mm. Then heat to 1020-1030℃ and hold for 2.5 min / mm. Immerse it in water for cooling at 0.2 min / mm. Remove it from the water when the surface temperature cools to 280-300℃.

[0017] S6. Spheroidizing Annealing: Slowly heat the forging blank to 400℃ and hold for 2.5 min / mm. Continue heating to 840-860℃ and hold for 2-3 min / mm. Then start cooling in the furnace at 20-25℃ / H. When it cools to 730-750℃, hold for 2-3 min / mm. Continue cooling to 520-540℃ and hold for 2-3 min / mm. Then cool to 300℃ and hold for 2-3 min / mm before removing from the furnace and cooling to room temperature.

[0018] The above-mentioned electroslag remelting process specifically includes the following steps:

[0019] (1) Transfer the steel ingot to an acid bath to remove the oxide impurities on the surface of the steel ingot. The acid completely soaks the steel ingot and keeps it soaked in the acid for 30-60 minutes.

[0020] (2) The steel ingot is placed into an electroplating chamber containing electroplating solution for electroplating treatment, and the electroplating chamber is evacuated to 5 x 10⁻⁶ m³ / h. -3 Pa, add electroplating solution and completely immerse the steel ingot, maintain electroplating treatment for 40-80 minutes; then vacuum transfer the steel ingot to an anti-oxidation treatment chamber, add anti-oxidation solution and completely immerse the steel ingot, maintain anti-oxidation treatment for 30-60 minutes, and allow the surface moisture of the steel ingot to air dry naturally.

[0021] (3) The steel ingot is sent to the electroslag remelting furnace for smelting. After being heated to a temperature of 1650-1720℃ and remelted, it is filtered through the slag pool. The steel ingot is heated and melted to form a molten metal. The molten metal flows into a resistance heating furnace equipped with an internal magnetic field coil. The internal magnetic field coil generates an internal static magnetic field inside the resistance heating furnace under the action of a constant current. At the same time, an external magnetic field coil is also set outside the resistance heating furnace. The external magnetic field coil generates an external static magnetic field outside the resistance heating furnace under the action of a constant current. The molten metal is kept in the resistance heating furnace for 60-80 minutes. The two static magnetic fields form a composite magnetic field, which promotes the flow of the molten metal in the resistance heating furnace. With the change of the magnetic field, circumferential convection heat transfer occurs and the molten metal is fully tumbled in the resistance heating furnace, finally obtaining a new steel ingot with a stable and homogeneous structure.

[0022] The acid solution is nitric acid with a concentration of 50-700%. The electroplating solution is composed of a mixture of deionized water, potassium cyanide, potassium carbonate and silver chloride. The antioxidant treatment solution is composed of a mixture of deionized water, potassium dichromate, potassium hydroxide and potassium carbonate.

[0023] The preparation method of this die-casting mold special steel has been further improved:

[0024] A salt bath treatment process is added between the ultrafine treatment and the spheroidizing annealing process. The forging blank is placed in a salt bath at a temperature of 400-450℃ and held for 60-80 minutes. Then the temperature is adjusted to 500℃-530℃ and held for 40-60 minutes. It is then self-cooled to 250℃. After the salt bath treatment process is completed, the forging blank is transferred to a nitriding chamber and nitrogen is injected. High-purity nitrogen of 3-6 bar is injected in a slow charging mode and held at a temperature of -50 to -100℃ for 10-30 minutes. Cooling is stopped when the core temperature of the mold reaches 100℃.

[0025] A salt bath includes a combination of two or more molten salts selected from sodium chloride, calcium chloride, ammonium nitrate, and potassium nitrite. In a two-salt combination, the mass ratio of the two molten salts is 1-3:1-3; in a three-salt combination, the mass ratio of the three molten salts is 1-3:1-3:3-5; and in a four-salt combination, the mass ratio of the three molten salts is 1-3:1-3:2-4:2-4.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1. This invention optimizes the chemical composition of traditional 4Cr5MoSiV material by appropriately reducing the carbon and silicon content and increasing the molybdenum content. Through the addition of ultrafine refining and electroslag remelting processes, the alloy concentration distribution and microstructure become more uniform, with a significant reduction in the difference between the core and surface microstructure. This effectively controls material purity and improves the spheroidizing annealing microstructure level. Therefore, it meets users' high requirements for the high-temperature strength and toughness of die-casting molds, ensuring long service life and creating high value.

[0028] 2. Increasing the nickel content in the composition of mold steel can further improve the steel's strength without significantly reducing its toughness; it lowers the brittle transition temperature, thus improving the steel's low-temperature toughness; it improves the steel's machinability and weldability; and it enhances the steel's corrosion resistance, making it resistant not only to acids but also to alkalis and atmospheric corrosion. Increasing the molybdenum (Mo) content in mold steel has the main characteristics of: molybdenum has a solid solution strengthening effect on ferrite; it improves the steel's hot strength and resistance to hydrogen erosion; and it improves the steel's hardenability. In particular, since die-casting molds are used at high temperatures, increasing the molybdenum content also improves the material's high-temperature strength and resistance to softening, stabilizes the mold's performance at high temperatures, and promotes and extends the mold's service life.

[0029] 3. This invention employs a process of acid immersion, electroplating, and anti-oxidation treatment before remelting the steel ingot. Acid immersion removes surface oxidation impurities caused by contact with air, creating favorable conditions for the high purity of the remelted molten metal. Following acid immersion, electroplating enhances the conductivity of the metals, creating favorable conditions for full convection of the remelted molten metal under a composite magnetic field. Then, the steel ingot is vacuum-transferred to an anti-oxidation treatment chamber, where an anti-oxidation solution is added and completely immersed, forming an anti-oxidation layer on the surface. This prevents secondary contact with air during the transfer of the ingot to the electroslag remelting furnace, thus avoiding surface oxidation impurities and further enhancing the high purity of the remelted molten metal.

[0030] 4. This invention incorporates magnetic field coils both inside and outside the resistance heating furnace. The inner magnetic field coil generates an internal static magnetic field within the furnace under the influence of a constant current. Simultaneously, an outer magnetic field coil is installed outside the furnace, generating an external static magnetic field under the same constant current. These two magnetic fields form a composite magnetic field, promoting the flow of molten metal within the furnace. The changing magnetic fields facilitate circumferential convective heat transfer, causing the molten metal to tumble thoroughly within the furnace, ultimately resulting in a new steel ingot with a stable and homogeneous microstructure. By employing a composite magnetic field, the molten metal undergoes more thorough circumferential convective heat transfer within the furnace, promoting uniformity of the circumferential temperature field, further improving the microstructure of the die steel, and enhancing its density, uniformity, and isotropic properties.

[0031] 5. This preparation method changes the traditional mold steel preparation process, which typically involves refining the forged billet and then re-annealing it in the furnace. This method optimizes the smelting-refining-electroslag remelting-forging-ultra-refining-spheroidizing annealing process by incorporating salt bath treatment and cryogenic treatment. The use of bath salt nitriding effectively solves the heat treatment problem of the metal surface, resulting in minimal deformation, shorter processing time, increased surface hardness, and the formation of an anti-corrosion salt-permeable layer. Furthermore, conventional techniques directly introduce nitrogen into the quenching furnace, which has the drawback of preventing the steel surface from cooling naturally, leading to a less dense surface structure and defects such as cracks and porosity. This method, through salt bath treatment and the formation of an anti-corrosion salt-permeable layer, followed by self-cooling and cryogenic treatment, improves the surface impact toughness of the steel by 30%. Moreover, the surface friction coefficient of the steel after cryogenic treatment is lower than that after conventional heat treatment, and the friction curve is smoother, significantly improving the wear resistance of the steel. After cryogenic treatment, the steel has a dense and fine structure, without defects such as cracks and pores that are visible to the naked eye.

[0032] To more clearly illustrate the features and effects of the present invention, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. Attached Figure Description

[0033] Figure 1 Micrograph of uniformly distributed dot-shaped and spherical pearlite in the annealed structure of Example 1 (etched at 50X with 5% nitric acid alcohol solution).

[0034] Figure 2 Micrograph of uniformly distributed dot-shaped and spherical pearlite in the annealed structure of Example 1 (etched at 500X with 5% nitric acid alcohol solution).

[0035] Figure 3 Image of uniformly distributed dot-shaped and spherical pearlite in the annealed structure of Example 2 (etched at 50X with 5% nitric acid alcohol solution);

[0036] Figure 4 Micrograph of uniformly distributed dot-shaped and spherical pearlite in the annealed structure of Example 2 (etched at 500X with 5% nitric acid alcohol solution).

[0037] Figure 5 Micrographs of dot-shaped and spherical pearlite uniformly distributed in the annealed structure of Example 3 (etched at 50X with 5% nitric acid alcohol solution).

[0038] Figure 6 Micrographs of dot-shaped and spherical pearlite uniformly distributed in the annealed structure of Example 3 (etched at 500X with 5% nitric acid alcohol solution).

[0039] Figure 7 Micrograph of inclusions in Example 1 obtained by electroslag remelting process (100X without etching);

[0040] Figure 8 Micrograph of inclusions obtained by electroslag remelting process in Example 2 (100X without etching);

[0041] Figure 9 Micrograph of inclusions obtained by electroslag remelting process in Example 3 (100X without etching). Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Example 1: This example provides a die-casting mold special steel comprising the following components by mass percentage: C: 0.36%; Si: 0.20%; Mn: 0.40%; P: ≦0.012%; S: ≦0.001%; ​​Cr: 4.90%; Ni: 0.40%; Mo: 2.20%; V: 0.50%; the remainder being Fe.

[0044] The preparation method of special steel for die casting molds includes the following steps:

[0045] S1. Smelting: According to the mass percentage of the components contained in the die casting mold steel, various alloy raw materials are put into the electric arc furnace and heated to gradually melt. Vacuum melting is carried out at a temperature of 1680℃ and held for 6 hours to make the temperature and composition of the whole furnace melt uniform.

[0046] S2, Refining: S1 is converted to LF ladle refining, VD vacuum degassing is followed by casting, and natural cooling is then performed.

[0047] Steel ingots;

[0048] S3, Electroslag Remelting: The steel ingot formed in S2 is heated to 1650-1720℃ using an ESR electroslag remelting equipment, then filtered through a slag pool and naturally cooled to generate a new steel ingot.

[0049] Transfer the steel ingot into an acid bath to remove oxide impurities from the surface of the steel ingot. The acid completely soaks the steel ingot and keeps it soaked in the acid for 30-60 minutes.

[0050] The steel ingot is placed into an electroplating chamber containing an electroplating solution for electroplating treatment, and the electroplating chamber is evacuated to a vacuum level of 5 x 10. - 3 Pa, add electroplating solution and completely immerse the steel ingot, maintain electroplating treatment for 40-80 minutes; then vacuum transfer the steel ingot to an anti-oxidation treatment chamber, add anti-oxidation solution and completely immerse the steel ingot, maintain anti-oxidation treatment for 30-60 minutes, and allow the surface moisture of the steel ingot to air dry naturally.

[0051] The steel ingot is sent to an electroslag remelting furnace for melting. After being heated to 1650-1720℃ and remelted, it is filtered through a slag pool. The heated and melted steel ingot forms molten metal, which flows into a resistance heating furnace equipped with an internal magnetic field coil. The internal magnetic field coil generates an internal static magnetic field inside the resistance heating furnace under the action of a constant current. At the same time, an external magnetic field coil is also set outside the resistance heating furnace. The external magnetic field coil generates an external static magnetic field outside the resistance heating furnace under the action of a constant current. The molten metal is kept in the resistance heating furnace for 60-80 minutes. The two static magnetic fields form a composite magnetic field, which promotes the flow of the molten metal in the resistance heating furnace. With the change of the magnetic field, circumferential convection heat transfer occurs, and the molten metal is fully tumbled in the resistance heating furnace, finally obtaining a new steel ingot with a stable and homogeneous microstructure.

[0052] S4. Forging process: The ESR steel ingot formed in S3 is heated to 1270℃ and held for 28 hours. Then, it is transferred to a forging press for three-dimensional surface forging. The final temperature is controlled at 830℃. After cooling to 300℃, it is transferred to ultrafine processing.

[0053] S5. Ultra-fine treatment: Slowly heat the S4 forging billet to 650℃ in a converter and hold for 8 hours. Continue to heat to 850℃ and hold for 8 hours. Then heat to 1020-1030℃ and hold for 8 hours. Place it in water to cool for 40 minutes. Remove it from the water when the surface temperature drops to 300℃.

[0054] S6. Salt bath nitriding treatment: Place the forging blank in a salt bath at a temperature of 400-450℃ and maintain for 60-80 minutes; then adjust the temperature to 500℃-530℃ and maintain for 40-60 minutes, and self-cool to 250℃. After the salt bath treatment process is completed, transfer the forging blank to a nitriding chamber and inject nitrogen gas. Inflate with 3-6 bar of high-purity nitrogen gas in slow inflation mode and maintain at a temperature of -50 to -100℃ for 10-30 minutes. Stop cooling when the core temperature of the mold reaches 100℃.

[0055] In this embodiment, the salt bath uses a mixture of ammonium nitrate and potassium nitrite, with a mass ratio of ammonium nitrate to potassium nitrite of 3. By using a mixture of ammonium nitrate and potassium nitrite and treating it in a salt bath, the interfacial tension, viscosity, and conductivity are increased, and an anti-corrosion salt penetration layer is formed on the surface.

[0056] S7. Spheroidizing annealing treatment: Slowly heat the forging blank to 400℃ and hold for 8 hours, then continue to heat to 840-860℃ and hold for 8 hours; then gradually cool down at 20-25℃ / h, hold at 730-750℃ for 8 hours, continue to cool to 520-540℃ and hold for 8 hours, then cool to 300℃ and hold for 8 hours before removing from the furnace and cooling to room temperature.

[0057] Example 2: This example provides a die-casting mold special steel comprising the following components by mass percentage: C: 0.37%; Si: 0.25%; Mn: 0.45%; P: ≦0.012%; S: ≦0.003%; Cr: 5.00%; Ni: 0.45%; Mo: 2.35%; V: 0.55%; the remainder being Fe.

[0058] The preparation method of special steel for die casting molds includes the following steps:

[0059] S1. Smelting: According to the mass percentage of the components contained in the die casting mold steel, various alloy raw materials are put into the electric arc furnace and heated to gradually melt. Vacuum melting is carried out at a temperature of 1685℃ and held for 6 hours to make the temperature and composition of the whole furnace melt uniform.

[0060] S2, Refining: S1 is converted to LF ladle refining, VD vacuum degassing is followed by casting, and natural cooling is used to form steel ingots;

[0061] S3, Electroslag Remelting: The steel ingot formed in S2 is heated to 1650-1720℃ using an ESR electroslag remelting equipment, then filtered through a slag pool and naturally cooled to generate a new steel ingot.

[0062] Transfer the steel ingot into an acid bath to remove oxide impurities from the surface of the steel ingot. The acid completely soaks the steel ingot and keeps it soaked in the acid for 30-60 minutes.

[0063] The steel ingot is placed into an electroplating chamber containing an electroplating solution for electroplating treatment, and the electroplating chamber is evacuated to a vacuum level of 5 x 10. - 3 Pa, add electroplating solution and completely immerse the steel ingot, maintain electroplating treatment for 40-80 minutes; then vacuum transfer the steel ingot to an anti-oxidation treatment chamber, add anti-oxidation solution and completely immerse the steel ingot, maintain anti-oxidation treatment for 30-60 minutes, and allow the surface moisture of the steel ingot to air dry naturally.

[0064] The steel ingot is sent to an electroslag remelting furnace for melting. After being heated to 1650-1720℃ and remelted, it is filtered through a slag pool. The heated and melted steel ingot forms molten metal, which flows into a resistance heating furnace equipped with an internal magnetic field coil. The internal magnetic field coil generates an internal static magnetic field inside the resistance heating furnace under the action of a constant current. At the same time, an external magnetic field coil is also set outside the resistance heating furnace. The external magnetic field coil generates an external static magnetic field outside the resistance heating furnace under the action of a constant current. The molten metal is kept in the resistance heating furnace for 60-80 minutes. The two static magnetic fields form a composite magnetic field, which promotes the flow of the molten metal in the resistance heating furnace. With the change of the magnetic field, circumferential convection heat transfer occurs, and the molten metal is fully tumbled in the resistance heating furnace, finally obtaining a new steel ingot with a stable and homogeneous microstructure.

[0065] S4. Forging process: The ESR steel ingot formed by S3 is heated to 1270℃ and held for 28 hours. Then, it is transferred to a forging press for three-dimensional surface forging. The final temperature is controlled at 830℃. After cooling to 300℃, it is transferred to ultra-fine processing.

[0066] S5. Ultra-fine treatment: Slowly heat the S4 forging billet to 650℃ in a converter and hold for 8 hours. Continue to heat to 850℃ and hold for 8 hours. Then heat to 1020-1030℃ and hold for 8 hours. Place it in water to cool for 40 minutes. Remove it from the water when the surface temperature drops to 300℃.

[0067] S6. Salt bath nitriding treatment: Place the forging blank in a salt bath at a temperature of 400-450℃ and maintain for 60-80 minutes; then adjust the temperature to 500℃-530℃ and maintain for 40-60 minutes, and self-cool to 250℃. After the salt bath treatment process is completed, transfer the forging blank to a nitriding chamber and inject nitrogen gas. Inflate with 3-6 bar of high-purity nitrogen gas in slow inflation mode and maintain at a temperature of -50 to -100℃ for 10-30 minutes. Stop cooling when the core temperature of the mold reaches 100℃.

[0068] In this embodiment, the salt bath uses a mixture of ammonium nitrate and potassium nitrite, with a mass ratio of 3:2 for the two molten salts. By using a mixture of ammonium nitrate and potassium nitrite and treating it in a salt bath, the interfacial tension, viscosity, and conductivity are increased, and an anti-corrosion salt penetration layer is formed on the surface.

[0069] S7. Spheroidizing annealing treatment: Slowly heat the forging blank to 400℃ and hold for 8 hours, then continue to heat to 840-860℃ and hold for 8 hours; then gradually cool it in the furnace at a rate of 20-25℃ / H, hold it at 730-750℃ for 8 hours, continue to cool it to 520-540℃ and hold for 8 hours, then cool it to 300℃ and hold for 8 hours before removing it from the furnace and cooling it to room temperature.

[0070] Example 3: This example provides a die-casting mold special steel comprising the following components by mass percentage: C: 0.38%; Si: 0.30%; Mn: 0.50%; P: ≦0.010%; S: ≦0.001%; ​​Cr: 5.10%; Ni: 0.50%; Mo: 2.50%; V: 0.60%; the remainder being Fe.

[0071] The preparation method of special steel for die casting molds includes the following steps:

[0072] S1. Smelting: According to the mass percentage of the components contained in the die casting mold steel, various alloy raw materials are put into the electric arc furnace and heated to gradually melt. Vacuum melting is carried out at a temperature of 1690℃ and held for 6 hours to make the temperature and composition of the whole furnace melt uniform.

[0073] S2, Refining: S1 is converted to LF ladle refining, VD vacuum degassing is followed by casting, and natural cooling is used to form steel ingots;

[0074] S3, Electroslag Remelting: The steel ingot formed in S2 is heated to 1650-1720℃ using an ESR electroslag remelting equipment, then filtered through a slag pool and naturally cooled to generate a new steel ingot.

[0075] Transfer the steel ingot into an acid bath to remove oxide impurities from the surface of the steel ingot. The acid completely soaks the steel ingot and keeps it soaked in the acid for 30-60 minutes.

[0076] The steel ingot is placed into an electroplating chamber containing an electroplating solution for electroplating treatment, and the electroplating chamber is evacuated to a vacuum level of 5 x 10. - 3 Pa, add electroplating solution and completely immerse the steel ingot, maintain electroplating treatment for 40-80 minutes; then vacuum transfer the steel ingot to an anti-oxidation treatment chamber, add anti-oxidation solution and completely immerse the steel ingot, maintain anti-oxidation treatment for 30-60 minutes, and allow the surface moisture of the steel ingot to air dry naturally.

[0077] The steel ingot is sent to an electroslag remelting furnace for melting. After being heated to 1650-1720℃ and remelted, it is filtered through a slag pool. The heated and melted steel ingot forms molten metal, which flows into a resistance heating furnace equipped with an internal magnetic field coil. The internal magnetic field coil generates an internal static magnetic field inside the resistance heating furnace under the action of a constant current. At the same time, an external magnetic field coil is also set outside the resistance heating furnace. The external magnetic field coil generates an external static magnetic field outside the resistance heating furnace under the action of a constant current. The molten metal is kept in the resistance heating furnace for 60-80 minutes. The two static magnetic fields form a composite magnetic field, which promotes the flow of the molten metal in the resistance heating furnace. With the change of the magnetic field, circumferential convection heat transfer occurs, and the molten metal is fully tumbled in the resistance heating furnace, finally obtaining a new steel ingot with a stable and homogeneous microstructure.

[0078] S4 forging process: The ESR steel ingot formed by S3 is heated to 1270℃ and held for 28 hours. Then, it is subjected to three-dimensional surface forging by a forging press. The final temperature is controlled at 830℃, and it is cooled to 300℃ for ultra-fine processing.

[0079] S5. Ultra-fine treatment: Slowly heat the S4 forging billet to 650℃ in a converter and hold for 8 hours. Continue to heat to 850℃ and hold for 8 hours. Then heat to 1020-1030℃ and hold for 8 hours. Place it in water to cool for 40 minutes. Remove it from the water when the surface temperature drops to 300℃.

[0080] S6. Salt bath nitriding treatment: Place the forging blank in a salt bath at a temperature of 400-450℃ and maintain for 60-80 minutes; then adjust the temperature to 500℃-530℃ and maintain for 40-60 minutes, and self-cool to 250℃. After the salt bath treatment process is completed, transfer the forging blank to a nitriding chamber and inject nitrogen gas. Inflate with 3-6 bar of high-purity nitrogen gas in slow inflation mode and maintain at a temperature of -50 to -100℃ for 10-30 minutes. Stop cooling when the core temperature of the mold reaches 100℃.

[0081] In this embodiment, the salt bath uses a mixture of ammonium nitrate and potassium nitrite, with a mass ratio of 3:1 for both molten salts. By using a mixture of ammonium nitrate and potassium nitrite and treating it in a salt bath, the interfacial tension, viscosity, and conductivity are increased, and an anti-corrosion salt penetration layer is formed on the surface.

[0082] S7. Spheroidizing annealing treatment: Slowly heat the forging blank to 400℃ and hold for 8 hours, then continue to heat to 840-860℃ and hold for 8 hours; then gradually cool it in the furnace at a rate of 20-25℃ / H, hold it at 730-750℃ for 8 hours, continue to cool it to 520-540℃ and hold for 8 hours, then cool it to 300℃ and hold for 8 hours before removing it from the furnace and cooling it to room temperature.

[0083] Experimental testing: The die-casting mold steel materials obtained in Examples 1-3 above were machined into different samples, and the performance of the three groups of samples was measured. The following data were obtained:

[0084] Table 1: Annealed microstructure and hardness test;

[0085]

[0086] Figure 1 Micrographs of dot-shaped and spherical pearlite uniformly distributed in the annealed structure of Example 1, after etching at 50X in 5% nitric acid alcohol solution;

[0087] Figure 2 Micrographs of dot-shaped and spherical pearlite uniformly distributed in the annealed structure of Example 1, after etching at 500X in 5% nitric acid alcohol solution;

[0088] Figure 3 The image shows the uniformly distributed dot-shaped pearlite and spherical pearlite in the annealed structure of Example 2, after etching at 50X in a 5% nitric acid alcohol solution;

[0089] Figure 4 Micrographs of dot-shaped and spherical pearlite uniformly distributed in the annealed structure of Example 2, after etching at 500X in 5% nitric acid alcohol solution;

[0090] Figure 5 Micrographs of dot-shaped and spherical pearlite uniformly distributed in the annealed structure of Example 3, after etching at 50X in 5% nitric acid alcohol solution;

[0091] Figure 6 Micrographs of dot-shaped and spherical pearlite uniformly distributed in the annealed structure of Example 3, after etching at 500X in 5% nitric acid alcohol solution;

[0092] Table 2: Results of inclusion test;

[0093]

[0094] Figure 7 Micrograph of inclusions in Example 1 obtained by electroslag remelting process (100X without etching);

[0095] Figure 8Micrograph of inclusions obtained by electroslag remelting process in Example 2 (100X without etching);

[0096] Figure 9 Micrograph of inclusions obtained by electroslag remelting process in Example 3 (100X without etching).

[0097] Table 3: Impact energy results of Charpy V-notch samples after oil quenching and three tempering processes;

[0098]

[0099] Table 4: Impact energy results of Charpy V-notch samples cut from 4Cr5MoSiV material after oil quenching and three tempering processes:

[0100]

[0101] The performance test results of the above three groups of samples all meet the expected requirements for uniform metallographic structure, well controlled inclusion content, and significantly improved impact toughness compared to 4Cr5MoSiV material, effectively ensuring the life of die-casting molds with high requirements and complex structures.

[0102] This invention optimizes the chemical composition of traditional 4Cr5MoSiV material by appropriately reducing carbon and silicon content and increasing molybdenum content. Through the addition of ultrafine refining and electroslag remelting processes, the alloy concentration distribution and microstructure become more uniform, with a significant reduction in the difference between the core and surface microstructure. This effectively controls material purity and improves the spheroidizing annealing microstructure level. Consequently, it meets users' high requirements for the high-temperature strength and toughness of die-casting molds, ensuring long service life and creating high value.

[0103] The increased nickel content in mold steel composition further enhances its strength without significantly reducing its toughness; it lowers the brittle transition temperature, thus improving low-temperature toughness; it improves machinability and weldability; and it enhances corrosion resistance, making it resistant not only to acids but also to alkalis and atmospheric corrosion. The increased molybdenum (Mo) content in mold steel has the key characteristics of solid solution strengthening of ferrite. It also improves the steel's hot strength and resistance to hydrogen erosion, and enhances its hardenability. In particular, for die-casting molds used at high temperatures, increasing the molybdenum content improves the material's high-temperature strength and resistance to softening, stabilizes the mold's performance at high temperatures, and promotes and extends the mold's service life.

[0104] This invention employs a process of acid immersion, electroplating, and anti-oxidation treatment before steel ingot remelting. Acid immersion removes surface oxidation impurities caused by air contact, creating favorable conditions for achieving high purity in the remelted molten metal. Following acid immersion, electroplating enhances the conductivity of the metals, facilitating full convection of the remelted molten metal under a composite magnetic field. The ingot is then vacuum-transferred to an anti-oxidation treatment chamber, where an anti-oxidation solution is added and completely immersed, forming an anti-oxidation layer on the surface. This prevents secondary air contact and the generation of surface oxidation impurities during the transfer of the ingot to the electroslag remelting furnace, further contributing to high purity in the remelted molten metal.

[0105] This invention incorporates magnetic field coils both inside and outside the resistance heating furnace. The inner magnetic field coil generates an internal static magnetic field within the furnace under the influence of a constant current. Simultaneously, an outer magnetic field coil is installed outside the furnace, generating an external static magnetic field under the same constant current. These two magnetic fields form a composite magnetic field, promoting the flow of molten metal within the furnace. The changing magnetic fields facilitate circumferential convective heat transfer, causing the molten metal to tumble thoroughly within the furnace, ultimately resulting in a new steel ingot with a stable and homogeneous microstructure. By employing this composite magnetic field, the molten metal undergoes more thorough circumferential convective heat transfer within the furnace, promoting uniformity of the circumferential temperature field and further improving the microstructure of the die steel, thereby enhancing its density, uniformity, and isotropic properties.

[0106] This preparation method departs from the traditional mold steel manufacturing process, which typically involves refining the forged billet and then re-annealing it in the furnace. This method optimizes the smelting-refining-electroslag remelting-forging-ultra-refining-spheroidizing annealing process by incorporating salt bath treatment and cryogenic treatment. The use of bath salt nitriding effectively solves the heat treatment problem of metal surfaces, resulting in minimal deformation, shorter processing time, increased surface hardness, and the formation of an anti-corrosion salt-permeable layer. Furthermore, conventional techniques directly introduce nitrogen gas into the quenching furnace, which has the drawback of preventing the steel surface from cooling naturally, leading to a less dense surface structure and defects such as cracks and porosity. This method, through salt bath treatment and the formation of an anti-corrosion salt-permeable layer, followed by self-cooling and cryogenic treatment, improves the surface impact toughness of the steel by 30%. Moreover, the surface friction coefficient of the steel after cryogenic treatment is lower than that after conventional heat treatment, and the friction curve is smoother, significantly improving the wear resistance of the steel. After cryogenic treatment, the steel has a dense and fine structure, without defects such as cracks and pores that are visible to the naked eye.

[0107] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any person skilled in the art can make many possible variations and modifications to the technical solution of the present invention, or modify it into equivalent embodiments, without departing from the scope of the present invention's technical solution. Therefore, all equivalent changes made based on the shape, structure, and principle of the present invention without departing from the scope of the present invention's technical solution should be covered within the protection scope of the present invention.

Claims

1. A method for preparing special steel for die casting molds, characterized in that, Includes the following steps: S1, Smelting: Includes the following components in mass percentage: C: 0.36%-0.38%; Si: 0.20%-0.30%; Mn: 0.40%-0.50%; P: ≤0.010%; S: ≦0.001%; ​​Cr: 4.90%-5.10%; Ni: 0.40%-0.50%; Mo: 2.2%-2.5%; V: 0.50%-0.60%; the remainder is Fe; according to the mass percentage of the components contained in the die casting mold steel, various alloy raw materials are put into the electric arc furnace and heated to gradually melt. Vacuum melting is carried out at a temperature controlled at 1650-1720℃ and held for 5-8 hours to make the temperature and composition of the whole furnace melt uniform. S2, Refining: S1 is converted to LF ladle refining, VD vacuum degassing is followed by casting, and natural cooling is used to form steel ingots; S3. Electroslag remelting: includes the following steps: (1) Transfer the steel ingot to an acid bath to remove the oxide impurities on the surface of the steel ingot. The acid completely soaks the steel ingot and keeps it soaked in the acid for 30-60 minutes. (2) The steel ingots after removing oxide impurities are placed into an electroplating chamber containing electroplating solution for electroplating treatment, and the electroplating chamber is evacuated to 5 x 10⁻⁶ m³ / h. -3 Pa, add electroplating solution and completely immerse the steel ingot, maintain electroplating treatment for 40-80 minutes; then vacuum transfer the steel ingot to an anti-oxidation treatment chamber, add anti-oxidation solution and completely immerse the steel ingot, maintain anti-oxidation treatment for 30-60 minutes, and allow the surface moisture of the steel ingot to air dry naturally. (3) The steel ingot is sent to the electroslag remelting furnace for smelting. After being heated to a temperature of 1650-1720℃ and remelted, it is filtered through the slag pool. The steel ingot is heated and melted to form a molten metal. The molten metal flows into a resistance heating furnace equipped with an internal magnetic field coil. The internal magnetic field coil generates an internal static magnetic field inside the resistance heating furnace under the action of a constant current. At the same time, an external magnetic field coil is also set outside the resistance heating furnace. The external magnetic field coil generates an external static magnetic field outside the resistance heating furnace under the action of a constant current. The two static magnetic fields form a composite magnetic field, which promotes the flow of the molten metal in the resistance heating furnace. With the change of the magnetic field, circumferential convection heat transfer occurs and the molten metal is fully tumbled in the resistance heating furnace, finally obtaining a new steel ingot with a stable and homogeneous structure. S4. Forging process: The steel ingot formed by S3 is heated to 1260-1280℃ and held for 25-30 hours. Then it is transferred to a forging press for three-dimensional surface forging. The final temperature is controlled at 800-850℃. After cooling to 200-300℃, it is transferred to ultrafine processing. S5. Ultra-fine treatment: Slowly heat the S4 forging billet to 650℃ in a converter and hold for 2.5 min / mm. Continue heating to 850℃ and hold for 2.5 min / mm. Continue heating to 1020-1030℃ and hold for 2.5 min / mm. Immerse it in water for cooling at 0.2 min / mm. Remove it from the water when the surface temperature cools to 280-300℃. A salt bath treatment process is added between the ultrafine treatment and the spheroidizing annealing. The forging blank is placed in a salt bath at a temperature of 400-450℃ and held for 60-80 minutes; then the temperature is adjusted to 500℃-530℃ and held for 40-60 minutes, and then self-cooled to 250℃; the salt bath includes two or more molten salts selected from sodium chloride, calcium chloride, ammonium nitrate, and potassium nitrite. After the salt bath treatment process is completed, the forging blank is transferred to the nitriding chamber and nitrogen is injected. High-purity nitrogen of 3-6 bar is injected in slow inflation mode and maintained at a temperature of -50 to -100℃ for 10-30 minutes. Cooling is stopped when the core temperature of the mold reaches 100℃. S6. Spheroidizing Annealing: Slowly heat the S5 forging blank to 400℃ and hold for 2.5 min / mm. Continue heating to 840-860℃ and hold for 2-3 min / mm. Then start cooling at 20-25℃ / h. When it cools to 730-750℃, hold for 2-3 min / mm. Continue cooling to 520-540℃ and hold for 2-3 min / mm. Continue cooling to 300℃ and hold for 2-3 min / mm before removing from the furnace and cooling to room temperature.

2. The method for preparing special steel for die casting molds according to claim 1, characterized in that, It comprises the following components by mass percentage: C: 0.38%; Si: 0.30%; Mn: 0.50%; P: ≦0.010%; S: ≦0.001%; ​​Cr: 5.10%; Ni: 0.50%; Mo: 2.50%; V: 0.60%; with the remainder being Fe.

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