A hot work die steel for high-toughness, high-uniformity, high-purity and large-size die casting and its preparation method

Abstract

The present invention relates to a preparation method of a hot work die steel for large-size die casting with high toughness, high uniformity, high purity, which aims to prepare qualified large-size hot work die flat steel by optimizing electrode blank smelting, electroslag remelting, forging and heat treatment, and strictly controlling the production process. The purity of the electrode blank is greatly improved by smelting the electrode blank from molten iron + scrap steel; the electroslag remelting process is optimized to improve the inclusion removal ability and reduce casting segregation; segregation is improved, and the density and uniformity are increased through ultra-high temperature diffusion and three-direction forging processes; the network carbide and austenite grain size are controlled by isothermal normalizing after forging; the solution treatment and spheroidizing annealing are used to control the dissolution and precipitation of carbides to improve the uniformity; the uniform distribution of alloying elements in the steel and sufficient deformation of the billet core are ensured, and the network carbide is eliminated and the austenite grains are refined by combining post-forging cooling and isothermal normalizing; the reasonable application of the solution treatment + spheroidizing annealing process ensures the carbide uniformity and spheroidizing uniformity.

Description

Technical Field

[0001] The present invention relates to a die steel and a preparation method thereof, in particular to a hot work die steel for die casting with high toughness, high uniformity, high purity and large size, and a preparation method thereof. Background Art

[0002] According to the "Energy Saving and New Energy Vehicle Technology Roadmap Figure 2 .0", in 2035, the fuel consumption of traditional energy passenger cars in China needs to be reduced to 4.0 L / km; the fuel consumption of passenger cars (including new energy) needs to be reduced to 2.0 L / km; the fuel consumption of trucks and buses needs to be reduced by at least 15% and 20% respectively compared with 2019. According to the data of "Bühler China", for every 100 kg reduction in the curb weight of a vehicle, the fuel consumption per 100 km can be reduced by 0.3 - 0.6 L. Strict fuel consumption and emission standards will give rise to a huge lightweight application market.

[0003] The current automotive industry is accelerating "new energy transformation", the penetration rate of new energy vehicles is rising rapidly, and the automotive lightweight market will accelerate its expansion. Some models even adopt an all-aluminum body design to pursue the ultimate lightweight effect, such as Tesla Model 3, Model Y, Audi A8, R8, Rolls-Royce Phantom, Mercedes-Benz SLS, Honda NSX, Jaguar XFL, NIO ES8, etc.

[0004] The demand for all-aluminum bodies has promoted the large-scale development of the die-casting industry. Due to the large-scale of die-casting machines and the corresponding large size of the supporting molds, the size and thickness of the die-casting die steel materials required reach 500 - 750 mm, the width is 1000 - 1500 mm, and the single weight reaches 10 - 18 t. At the same time, due to the substantial increase in the value of a single mold, in order to ensure the safety of the mold, higher requirements are put forward for material purity, tissue uniformity, strength and toughness, etc. Due to the need for producing large-sized and high-quality die steel, a forging machine with a capacity of 5000 t or more, an argon gas-protected electroslag remelting furnace with a capacity of more than 15 t and supporting heat treatment equipment have become necessary configurations. This has led to the situation that most domestic die steel production enterprises currently do not have the relevant capabilities, and there are the following problems in the production process:

[0005] Smelting process: During the smelting of electrode billets and electroslag remelting ingots, the composition segregation is serious;

[0006] Heating: Long-time and ultra-high temperature lead to serious grain coarsening;

[0007] Forging: Insufficient forging equipment capacity leads to poor forging penetration;

[0008] Heat treatment: During the heat treatment quenching and annealing processes, it is not easy to control the heating and cooling uniformity.

[0009] Meeting comprehensive performance standards: The V-notch impact energy and spheroidized structure are not easy to meet the standards. Summary of the Invention

[0010] The object of the present invention is to provide a preparation method of a hot work die steel for large-size die casting with high toughness, high uniformity, high purity, which aims to prepare qualified large-size hot work die flat steel by optimizing the processes of electrode blank smelting, electroslag remelting, forging and heat treatment, and strictly controlling the production process.

[0011] To achieve the above object of the invention, a preparation method of a hot work die steel for large-size die casting with high toughness, high uniformity, high purity is as follows:

[0012] S1. Electric furnace smelting:

[0013] Charge the blast furnace hot metal, sheared furnace charge and tundish residue steel according to the component content of the hot work die steel, where the weight ratio of the blast furnace hot metal is ≥80%, add CaO, CaF2, aluminum blocks, dolomite, and melt and smelt in the electric furnace; after slag skimming, add CaO, cleaning agent, aluminum blocks and then tap the steel, and the tapping temperature is ≥1620°C;

[0014] Among them, the component content of the hot work die steel is, by mass percentage, C 0.33% - 0.38%, Mn 0.30% - 0.50%, Si 0.80% - 1.10%, S ≤ 0.001%, P ≤ 0.008%, Cr 4.90% - 5.10%, Mo 1.30% - 1.60%, V 0.50% - 0.70%, Ni 0.25% - 0.45%, and the rest is Fe, and the residual gas content is, H ≤ 1.5 ppm, O ≤ 10 ppm, N ≤ 60 ppm; the residual harmful elements Cu ≤ 0.05%, Ti, Sn, Pb are all ≤ 0.002%; low C, low P and high Ni improve toughness, and low Si can reduce the solidification segregation of molten steel and improve uniformity.

[0015] S2. Ladle furnace refining:

[0016] Heat the ladle in the LF ladle furnace, use Fe-Si powder and Si-C powder for slag surface diffusion deoxidation, and add an appropriate amount of CaO; the refining time is ≥35 minutes, and the white slag time is ≥20 minutes;

[0017] S3. Vacuum furnace refining:

[0018] The vacuum furnace is pumped step by step, the vacuum degree reaches 67 Pa, and the vacuum holding time is ≥10 minutes to ensure that the residual gas H ≤ 1.5 ppm. After vacuum degassing, sample and analyze. After the composition is qualified, blow argon until the ladle is lifted, and the soft blowing argon time is ≥15 min;

[0019] S4. Continuous casting electrode blank:

[0020] The superheat degree for the first heat is 20 - 35°C, and for the continuous casting heats is 18 - 30°C; the continuous casting speed is 0.12 - 0.14 m / min;

[0021] S5. Annealing of the electrode billets:

[0022] The electrode billets obtained by continuous casting in step S4 are flame cut into segments and then annealed. The trolley furnace is preheated to ≥500°C for ≥2 h, slowly heated to 860 - 880°C for recrystallization annealing, the holding time is ≥35 h, and then cooled in the furnace to 300°C and taken out of the furnace;

[0023] After continuous casting, it cools and solidifies into martensite, bainite, pearlite, and retained austenite. It is reheated to the temperature range of AC1 - AC3 and held for a long time. The original structure is re - transformed into austenite + carbide, and then slowly cooled to re - transform into a mixed structure of martensite, bainite, pearlite, and retained austenite. Through the annealing of the electrode billets, the deformed and irregular - shaped austenite formed in the early stage is re - austenitized to form equiaxed austenite, which not only eliminates the non - uniformity but also eliminates the tissue stress, avoiding cracking due to excessive stress after subsequent cooling to room temperature.

[0024] S6. Electroslag remelting:

[0025] Using the electrode billets obtained in step S5, the surface is turned smooth by a lathe, and smelting is carried out using a quaternary slag system of calcium fluoride, alumina, calcium oxide, and magnesium oxide to obtain an electroslag ingot, and then the power is cut off and furnace - cooled for 80 - 100 min and sent to the forging process;

[0026] S7. Forging:

[0027] The electroslag ingots obtained in step S6 are subjected to ultra - high - temperature homogenization diffusion control. The heating temperature for ultra - high - temperature homogenization is ≥1280°C, and diffusion homogenization is carried out by holding for 30 - 45 h, and then it is subjected to multi - directional upsetting and drawing for blooming in the X, Y, and Z directions, and finally drawn to the finished product size in one heat;

[0028] S8. Isothermal normalizing treatment:

[0029] The forged materials obtained in step S7 are subjected to isothermal normalizing treatment;

[0030] S9. Ultra - refinement treatment.

[0031] Optionally, in step S2, for the adjustment of alloy components, one or several of ferromanganese, high - carbon ferrochrome, low - carbon ferrochrome, ferromolybdenum, ferrovanadium, and ferronickel are selected.

[0032] Preferably, in step S4, a three-stage electromagnetic stirring of a crystallizer section, a casting stream section, and a continuous casting end section is adopted, and two-stage water cooling is used; the surface temperature of the continuous casting billet is ≤800 °C. Through the three-stage electromagnetic stirring, the equiaxed crystal zone of the billet can be expanded, and the columnar crystal zone can be reduced. Expanding and refining the equiaxed crystal zone can increase the crack resistance of the billet, eliminate the central shrinkage cavity, reduce the central porosity and central segregation; improve the surface quality of the billet, which is conducive to preventing surface cracks; improve the inclusion distribution in the billet. Eliminate or reduce the accumulation of inclusions on the inner arc side of the billet; eliminate the shrinkage cavity of the billet, improve the central porosity and central segregation; change the dendritic morphology of the stirring zone in the billet.

[0033] Preferably, in step S6, a constant melting rate control is adopted for ingot molds of 15 tons to 25 tons: the starting melting rate value in the steady state stage of electroslag remelting is 10 - 13.5 kg / min, and the ending melting rate value is 8.0 - 10.0 kg / min.

[0034] The constant melting rate ensures the stability of metallurgical chemical reactions and the stability of molten steel quality throughout the smelting process;

[0035] The low melting rate controls the size of molten metal droplets to obtain a suitable specific surface area. It not only achieves sufficient contact and reaction between the pre-melted slag and the molten steel to achieve the purpose of removing inclusions, but also maintains the temperature of the molten steel pool, making it easy for inclusions in the molten pool to float to the slag-metal interface and be captured and removed by the slag.

[0036] However, the melting rate cannot be too low, otherwise the molten pool temperature will be too low, reducing the rate of metallurgical chemical reactions. At the same time, after the molten droplets become too large, the specific surface area will decrease, weakening the ability to remove inclusions. At the same time, the surface quality of the electroslag ingot will deteriorate significantly.

[0037] Specifically, in step S7, the heating temperature during forging is 1140 - 1170 °C, and the final deformation amount is ≥50%; the ultimate cooling control after forging is to cool the billet to a core temperature of ≤350 °C in one water cooling.

[0038] Specifically, in step S8, the specific operation of isothermal normalizing treatment is: heat the billet to 980 - 1030 °C, and the holding time is ≥25 h; after the holding is completed, air-cool, and when the center of the large surface of the billet cools to 450 - 550 °C, reheat it to 720 - 770 °C for holding, and the holding time is ≥50 h; the number of repetitions of the above process is ≥2 times; after the last holding is completed, cool it in the furnace to 300 °C and then take it out and air-cool.

[0039] In martensite, bainite, pearlite, and retained austenite in steel, when reheated to the AC3 temperature range and held for a long time, the original structure will transform back into austenite again. Low-melting carbides will dissolve into the austenite, while high-melting vanadium carbides will remain. Subsequently, after air cooling, it is mainly supercooled austenite + high-melting vanadium carbides. When held at 720 - 770°C in the pearlite range for a long time, the pearlite phase transformation is completed, and the main structures are pearlite, retained austenite, and high-melting vanadium carbides. Through cyclic isothermal normalizing, the austenite grain size can be gradually refined, the structure heredity can be eliminated, and the carbides can be homogenized.

[0040] Furthermore, in step S8, after the heat preservation ends, a fog cannon blower is used for air cooling. If the cooling rate is too fast, it is easy to form bainite and martensite, resulting in the retention of structure heredity and failing to achieve the effect of grain refinement; if the cooling rate is too slow, a large amount of network carbides will precipitate. By selecting a blower with an appropriate power, the cooling rate of the billet when it leaves the furnace is ensured to be within an appropriate range. While reducing the structure phase transformation, the formation of network carbides is inhibited.

[0041] Specifically, in step S9, the specific steps of the ultra-refinement treatment are as follows;

[0042] (A) Solution treatment: Place the workpiece obtained in step S8 into a heating furnace and heat it up to 980 - 1030°C along with the furnace. After heat preservation, adopt extreme cooling control: direct water cooling method, and the maximum surface temperature rise after cooling is 180 - 220°C; in martensite, bainite, pearlite, and retained austenite in steel, when reheated to a temperature above AC3 and held for a long time, the original structure will transform back into austenite again. Low-melting carbides will dissolve into the austenite, while high-melting vanadium carbides will remain. After quenching out of the furnace, martensite, bainite, and retained austenite are formed;

[0043] (B) Spheroidizing annealing: Place the workpiece obtained in step (A) into an annealing furnace, heat it to 860 - 880°C, hold for 20 - 30h, after the heat preservation ends, cool it down to 740 - 760°C, and hold for 40 - 60h for spheroidizing annealing. After the heat preservation ends, cool it in the furnace to below 300°C and then take it out for air cooling; the spheroidizing treatment is to heat the original structures of martensite, bainite, and retained austenite to the AC1 - AC3 temperature range to form austenite and undissolved carbides; at a temperature below AC1, the lamellar carbides begin to fuse and form spherical carbides.

[0044] Another object of the present invention is to provide a hot work die steel for large-size die casting with high toughness, high uniformity, high purity, and high toughness prepared by the above preparation method.

[0045] A preparation method of a hot work die steel for large-size die casting with high toughness, high uniformity, high purity, and high toughness according to the present invention has the following advantages compared with the prior art:

[0046] (1) This process significantly improves the purity of the electrode blank by smelting the electrode blank from hot metal (smelted from iron ore) + scrap steel.

[0047] (2) Optimize the electroslag remelting process to improve the inclusion removal ability and reduce casting segregation.

[0048] (3) Improve segregation through ultra-high temperature diffusion and three-way forging processes, and enhance the density and uniformity; control the network carbide and austenite grain size by isothermal normalizing after forging.

[0049] (4) Solution treatment and spheroidizing annealing control the dissolution and precipitation of carbides to improve uniformity, ensure the uniform distribution of alloying elements in the steel and sufficient deformation of the billet core. At the same time, combined with post-forging cooling and isothermal normalizing to eliminate network carbides and refine austenite grains.

[0050] (5) The reasonable application of the solution treatment + spheroidizing annealing process ensures the carbide uniformity and spheroidizing uniformity. Finally, it ensures the uniform microstructure and high impact performance of the product in the annealed state, provides good conditions for subsequent quenching and tempering treatment, and this product can be widely used in the production and manufacturing of large-sized hot die casting molds for large aluminum alloy castings. Description of the Drawings

[0051] Figure 1 Microstructure morphology 1 of the die steel obtained in Example 1;

[0052] Figure 2 Microstructure morphology 2 of the die steel obtained in Example 1;

[0053] Figure 3 Microstructure morphology 3 of the die steel obtained in Example 1;

[0054] Figure 4 Microstructure morphology 1 of the die steel obtained in Example 2;

[0055] Figure 5 Microstructure morphology 2 of the die steel obtained in Example 2;

[0056] Figure 6 Microstructure morphology 3 of the die steel obtained in Example 2;

[0057] Figure 7 Microstructure morphology 1 of the die steel obtained in Example 3;

[0058] Figure 8 Microstructure morphology 2 of the die steel obtained in Example 3;

[0059] Figure 9 Microstructure morphology 3 of the die steel obtained in Example 3;

[0060] Figure 10The microscopic structure morphology 1 of the die steel obtained in Example 4;

[0061] Figure 11 The microscopic structure morphology 2 of the die steel obtained in Example 4;

[0062] Figure 12 The microscopic structure morphology 3 of the die steel obtained in Example 4;

[0063] Figure 13 The microscopic structure morphology 1 of the die steel obtained in Example 5;

[0064] Figure 14 The microscopic structure morphology 2 of the die steel obtained in Example 5;

[0065] Figure 15 The microscopic structure morphology 3 of the die steel obtained in Example 5. Detailed implementation manners

[0066] The following further describes the present invention in conjunction with the accompanying drawings and specific embodiments.

[0067] Example 1

[0068] A hot work die steel for high - toughness, high - uniformity, high - purity, and large - size die - casting, according to mass fraction, is basically composed of the elements shown in Tables 1 - 1 and 1 - 2:

[0069] Table 1 - 1 Chemical composition %

[0070] C Mn Si S P Cr Mo V Ni 0.35 0.45 1.0 0.001 0.006 5.01 1.45 0.55 0.36

[0071] Note: The rest is Fe

[0072] Table 1 - 2 Harmful elements % and residual gases (ppm)

[0073] Cu Ti Sn Pb H O N 0.03 0.0015 0.001 0.002 1.2 9 55

[0074] A preparation method of a hot work die steel for high - toughness, high - uniformity, high - purity, and large - size die - casting is as follows:

[0075] S1. Electric furnace smelting:

[0076] Charge the blast furnace hot metal (proportion 85%) and alloy materials according to the component content of the hot work die steel, add CaO, CaF2, aluminum blocks, and dolomite, and melt and smelt in an electric furnace; after slag - skimming, add CaO, cleaning agent, and aluminum blocks and then tap the steel, with the tapping temperature of 1640 °C;

[0077] S2. Ladle furnace refining:

[0078] The ladle furnace of LF is seated and heated. Fe-Si powder and Si-C are used for diffusion deoxidation on the slag surface, and an appropriate amount of CaO is added. The refining time is 40 minutes, and the white slag time is 23 minutes. Metal manganese, high-carbon ferrochrome, low-carbon ferrochrome, ferromolybdenum, ferrovanadium, etc. are selected for alloy composition adjustment.

[0079] S3. Vacuum refining furnace refining:

[0080] The vacuum refining furnace pumps step by step until the vacuum degree reaches 67 Pa, and the vacuum holding time is 14 minutes to ensure that the residual gas H ≤ 1.5 ppm. After the vacuum degassing is completed, sampling and analysis are carried out. After the composition is qualified, argon is blown until the ladle is lifted, and the soft blowing time of argon is 20 min;

[0081] S4. Continuous casting of electrode billets:

[0082] The superheat is 25 °C; the continuous casting speed is 0.13 m / min. Three-stage electromagnetic stirring, namely the mold section, the casting stream section, and the end of continuous casting, and two-stage water cooling are adopted; the surface temperature of the continuous casting billet is 760 - 790 °C.

[0083] S5. Annealing of electrode billets:

[0084] The electrode billets continuously cast in step S4 are flame cut into segments and then annealed. The trolley furnace is preheated to 500 °C for 3 h, slowly heated to 880 °C for recrystallization annealing, the holding time is 40 h, and then cooled in the furnace to 300 °C and then taken out of the furnace.

[0085] S6. Electro-slag remelting:

[0086] The electrode billets obtained in step S5 are used. The surface is turned smooth by a lathe, and a four-component slag system of calcium fluoride, alumina, calcium oxide, and magnesium oxide is used for smelting (where the impurities are silicon dioxide 0.54%, ferrous oxide 0.15%, and moisture 0.005%). For the 15 t ingot type, constant melting rate control is adopted: the starting melting rate value at the steady state stage of electro-slag smelting is 11.5 kg / min, and the ending melting rate value is 9.0 kg / min to obtain an electro-slag ingot, and then the power is cut off and the furnace is cooled for 90 min and then sent to the forging process;

[0087] S7. Forging:

[0088] The electro-slag ingot obtained in step S6 is subjected to ultra-high temperature diffusion control: the heating temperature is 1290 °C, and it is held for 40 h for diffusion homogenization, and then it is upset and drawn in three directions of X, Y, and Z for blooming, and finally drawn to the finished product size in one heat, and the heating temperature is 1150 °C, and the deformation amount in the last heat is 52%. After forging, extreme cooling control is carried out, and it is water-cooled once to the large surface temperature of the billet of 240 °C;

[0089] S8. Isothermal normalizing treatment:

[0090] Isothermally normalize the forging material obtained in step S7: Heat the blank to 1000°C and hold for 25 hours; after holding, use a fog gun blower for air cooling. After the large surface center cools to 500°C, reheat it to 740°C for holding, and the holding time is 50 hours. Repeat the above process 3 times. After the last holding is completed, cool it in the furnace to 300°C and then take it out of the furnace for air cooling.

[0091] S9. Solution treatment:

[0092] Place the workpiece obtained in step S8 into a heating furnace and heat it up to 1020°C with the furnace. After holding, adopt extreme cooling control: direct water cooling method, and the maximum temperature rise of the large surface after cooling is 205°C;

[0093] S10. Spheroidizing annealing:

[0094] Place the workpiece obtained in step S9 into an annealing furnace, heat it to 880°C, hold for 30 hours, after holding, cool it to 740°C, and the holding time is 60 hours for spheroidizing annealing. After holding, cool it in the furnace to below 300°C and then take it out of the furnace for air cooling.

[0095] Example 2

[0096] A hot work die steel for high - toughness, high - uniformity, high - purity, and large - size die casting, by mass fraction, is basically composed of the elements shown in Tables 2 - 1 and 2 - 2:

[0097] Table 2 - 1 Chemical composition %

[0098] C Mn Si S P Cr Mo V Ni 0.36 0.44 0.97 0.001 0.006 5.00 1.46 0.57 0.38

[0099] Note: The rest is Fe

[0100] Table 2 - 2 Harmful elements % and residual gases (ppm)

[0101] Cu Ti Sn Pb H O N 0.03 0.0017 0.001 0.002 1.5 8 52

[0102] A preparation method of a hot work die steel for high - toughness, high - uniformity, high - purity, and large - size die casting is as follows:

[0103] S1. Electric furnace smelting:

[0104] Charge the blast furnace hot metal (mixing ratio 85%) and alloy materials according to the component content of the hot work die steel, add CaO, CaF2, aluminum blocks, and dolomite, and melt and smelt in an electric furnace; after slag skimming, add CaO, purification accelerator, and aluminum blocks and then tap the steel, and the tapping temperature is 1640°C;

[0105] S2. Ladle furnace refining:

[0106] LF ladle refining furnace seat heating, using Fe-Si powder and Si-C slag surface diffusion deoxidation, adding an appropriate amount of CaO; refining time is 37 minutes, white slag time is 22 minutes; for alloy composition adjustment, metallic manganese, high-carbon ferrochrome, low-carbon ferrochrome, ferromolybdenum, ferrovanadium, etc. are selected.

[0107] S3. Vacuum refining furnace refining:

[0108] The vacuum refining furnace pumps step by step, the vacuum degree reaches 67 Pa, the vacuum holding time is 15 minutes, ensuring that the residual gas H ≤ 1.5 ppm. After the vacuum degassing is completed, sampling and analysis are carried out. After the composition is qualified, argon is blown until the ladle is lifted, and the soft blowing argon time is 20 min;

[0109] S4. Continuous casting electrode billet:

[0110] Superheat is 30 °C; continuous casting speed is 0.13 m / min, and three-stage electromagnetic stirring of the mold section, casting stream section, and continuous casting end section and two-stage water cooling are adopted; the surface temperature of the continuous casting billet is 760 - 790 °C.

[0111] S5. Electrode billet annealing:

[0112] The electrode billet obtained by continuous casting in step S4 is flame cut into segments and then annealed. The trolley furnace is preheated to 500 °C for 3 h, slowly heated to 880 °C for recrystallization annealing, the holding time is 40 h, and it is cooled in the furnace to 300 °C and then taken out of the furnace.

[0113] S6. Electroslag remelting:

[0114] Using the electrode billet obtained in step S5, the surface is turned smooth by a lathe, and smelting is carried out using a four-component pre-melted slag of high-purity calcium fluoride, alumina, calcium oxide, and magnesium oxide (where the impurities are silicon dioxide 0.54%, ferrous oxide 0.15%, and moisture 0.005%). For the 15 t ingot type, constant melting speed control is adopted: the starting melting speed value at the steady state stage of electroslag smelting is 11.5 kg / min, the ending melting speed value is 9.0 kg / min, an electroslag ingot is obtained, and then the power is cut off and the furnace is cooled for 90 min and then sent to the forging process;

[0115] S7. Forging:

[0116] The electroslag ingot obtained in step S6 is subjected to ultra-high temperature diffusion control: the heating temperature is 1290 °C, and it is held for 40 h for diffusion homogenization, and then it is upset and drawn in three directions of X, Y, and Z to open the billet. Finally, it is drawn to the finished product size in one heat, the heating temperature is 1150 °C, and the deformation amount in the last heat is 54%. After forging, extreme cooling control is carried out, and it is water-cooled once to the billet large surface temperature of 260 °C;

[0117] S8. Isothermal normalizing treatment:

[0118] The forged material obtained in step S7 is subjected to isothermal normalizing: the billet is heated to 1000 °C and held for 30 h; after the holding is completed, it is air-cooled using a fog cannon blower. After the center of the large surface is cooled to 500 °C, it is reheated to 740 °C and held for 60 h. The above process is repeated 3 times. After the last holding is completed, it is cooled in the furnace to 300 °C and then taken out of the furnace for air-cooling.

[0119] S9. Ultra-fine grain treatment:

[0120] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020 °C along with the furnace. After holding, extreme cooling control is adopted: direct water-cooling method, and the maximum return temperature of the large surface after cooling is 210 °C;

[0121] S10. Spheroidizing annealing:

[0122] The workpiece obtained in step S9 is placed in an annealing furnace, heated to 880 °C, held for 35 h, and after the holding is completed, it is cooled to 740 °C and held for 70 h for spheroidizing annealing. After the holding is completed, it is cooled in the furnace to below 300 °C and then taken out of the furnace for air-cooling.

[0123] Example 3

[0124] A hot work die steel for high-ductility, high-uniformity, high-purity, and large-size die casting, by mass fraction, is basically composed of the elements shown in Tables 3-1 and 3-2:

[0125] Table 3-1 Chemical composition %

[0126] C Mn Si S P Cr Mo V Ni 0.35 0.44 0.98 0.001 0.008 5.05 1.48 0.60 0.35

[0127] Note: The rest is Fe

[0128] Table 3-2 Harmful elements % and residual gases (ppm)

[0129]

[0130]

[0131] A preparation method of a hot work die steel for high-ductility, high-uniformity, high-purity, and large-size die casting, the preparation method is as follows:

[0132] S1. Electric furnace smelting:

[0133] According to the component content of the hot work die steel, blast furnace hot metal (mixing ratio 85%) and alloy materials are proportioned, CaO, CaF2, aluminum blocks, and dolomite are added, and melted and smelted in an electric furnace; after slag skimming, CaO, cleaning agent, and aluminum blocks are added and then tapped, and the tapping temperature is 1640 °C;

[0134] S2. Ladle furnace refining:

[0135] LF ladle refining furnace seat heating, using Fe-Si powder and Si-C slag surface diffusion deoxidation, adding an appropriate amount of CaO; refining time is 38 minutes, white slag time is 25 minutes; for alloy composition adjustment, metallic manganese, high-carbon ferrochrome, low-carbon ferrochrome, ferromolybdenum, ferrovanadium, etc. are selected.

[0136] S3. Vacuum refining furnace refining:

[0137] The vacuum refining furnace pumps step by step, the vacuum degree reaches 67 Pa, the vacuum holding time is 17 minutes, ensuring that the residual gas H ≤ 1.5 ppm. After the vacuum degassing is completed, sampling and analysis are carried out. After the composition is qualified, argon is blown until the ladle is lifted, and the soft blowing argon time is 20 min;

[0138] S4. Continuous casting of electrode billets:

[0139] Superheat is 35°C; continuous casting speed is 0.13 m / min, and three-stage electromagnetic stirring of the mold section, casting stream section, and continuous casting end section and two-stage water cooling are adopted; the surface temperature of the continuous casting billet is 760 - 800°C.

[0140] S5. Annealing of electrode billets:

[0141] The electrode billets continuously cast in step S4 are flame cut into segments and then annealed. The trolley furnace is preheated to 500°C for 3 h, slowly heated to 880°C for recrystallization annealing, the holding time is 40 h, and then cooled in the furnace to 300°C and taken out of the furnace.

[0142] S6. Electroslag remelting:

[0143] Using the electrode billets obtained in step S5, the surface is turned smooth by a lathe, and smelting is carried out using a four-component pre-melted slag of high-purity calcium fluoride, alumina, calcium oxide, and magnesium oxide (where the impurity silicon dioxide is 0.54%, ferrous oxide is 0.15%, and moisture is 0.005%). For the 15t ingot type, constant melting speed control is adopted: the starting melting speed value at the steady state stage of electroslag smelting is 11.5 kg / min, the ending melting speed value is 9.0 kg / min, to obtain an electroslag ingot, and then the power is cut off and the furnace is cooled for 90 min and then sent to the forging process;

[0144] S7. Forging:

[0145] The electroslag ingots obtained in step S6 are subjected to ultra-high temperature diffusion control: the heating temperature is 1290°C, and diffusion homogenization is carried out for 40 h, and then multi-directional upsetting and drawing are carried out in the X, Y, and Z directions to open the billet, and finally the billet is drawn to the finished product size in one heat, the heating temperature is 1150°C, and the deformation amount in the last heat is 51%. After forging, extreme cooling control is carried out, and the billet is directly water-cooled to the large surface temperature of 280°C at one time;

[0146] S8. Isothermal normalizing treatment:

[0147] The forged material obtained in step S7 is subjected to isothermal normalizing: the billet is heated to 1000 °C and held for 33 h; after the holding is completed, it is air-cooled using a fog gun blower. After the center of the large surface is cooled to 500 °C, it is reheated to 740 °C and held for 66 h. The above process is repeated 3 times. After the last holding is completed, the furnace is cooled to 300 °C and then taken out of the furnace for air-cooling.

[0148] S9. Ultra-fine grain treatment:

[0149] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020 °C with the furnace. After holding, extreme cooling control is adopted: direct water-cooling method, and the maximum temperature rise of the large surface after cooling is 200 °C;

[0150] S10. Spheroidizing annealing:

[0151] The workpiece obtained in step S9 is placed in an annealing furnace, heated to 880 °C, held for 35 h, cooled to 740 °C after the holding is completed, held for 70 h for spheroidizing annealing, and after the holding is completed, the furnace is cooled to below 300 °C and then taken out of the furnace for air-cooling.

[0152] Example 4

[0153] A hot work die steel for high-ductility, high-uniformity, high-purity, large-size die casting, by mass fraction, is basically composed of the elements shown in Tables 4-1 and 4-2:

[0154] Table 4-1 Chemical composition %

[0155] C Mn Si S P Cr Mo V Ni 0.36 0.45 1.02 0.001 0.007 4.98 1.38 0.57 0.32

[0156] Note: The rest is Fe

[0157] Table 4-2 Harmful elements % and residual gases (ppm)

[0158] Cu Ti Sn Pb H O N 0.05 0.0017 0.001 0.002 1.3 9 59

[0159] A preparation method of a hot work die steel for high-ductility, high-uniformity, high-purity, large-size die casting is as follows:

[0160] S1. Electric furnace smelting:

[0161] According to the component content of the hot work die steel, blast furnace hot metal (proportion 85%) and alloy materials are proportioned, CaO, CaF2, aluminum blocks, and dolomite are added, and melted and smelted in an electric furnace; after slag skimming, CaO, a purification promoter, and aluminum blocks are added and then tapped, and the tapping temperature is 1650 °C;

[0162] S2. Ladle furnace refining:

[0163] The LF ladle refining furnace is seated and heated, Fe-Si powder and Si-C are used for slag surface diffusion deoxidation, and an appropriate amount of CaO is added; the refining time is 37 minutes, and the white slag time is 23 minutes; for alloy composition adjustment, metallic manganese, high-carbon ferrochrome, low-carbon ferrochrome, ferromolybdenum, ferrovanadium, etc. are selected.

[0164] S3. Vacuum refining furnace refining:

[0165] The vacuum refining furnace pumps step by step, the vacuum degree reaches 67 Pa, the vacuum holding time is 19 minutes, ensuring that the residual gas H ≤ 1.5 ppm. After the vacuum degassing is completed, sampling and analysis are carried out. After the composition is qualified, argon is blown until the ladle is lifted, and the soft argon blowing time is 24 min;

[0166] S4. Continuous casting of electrode billets:

[0167] The superheat is 25 °C; the continuous casting speed is 0.13 m / min, and three-stage electromagnetic stirring, namely the mold section, the casting stream section, and the end of the continuous casting, and two-stage water cooling are adopted; the surface temperature of the continuous casting billet is 760 - 800 °C.

[0168] S5. Annealing of electrode billets:

[0169] The electrode billets continuously cast in step S4 are flame cut into segments and then annealed. The trolley furnace is preheated to 500 °C for 3 h, slowly heated to 880 °C for recrystallization annealing, the holding time is 40 h, and it is taken out of the furnace after cooling to 300 °C in the furnace.

[0170] S6. Electroslag remelting:

[0171] Using the electrode billets obtained in step S5, the surface is turned smooth by a lathe, and a four-component pre-melted slag of high-purity calcium fluoride, alumina, calcium oxide, and magnesium oxide (where the impurities are silicon dioxide 0.54%, ferrous oxide 0.15%, and moisture 0.005%) is used for smelting. For the 15t ingot type, constant melting speed control is adopted: the starting melting speed value at the steady state stage of electroslag smelting is 11.5 kg / min, and the ending melting speed value is 9.0 kg / min to obtain an electroslag ingot, and then the power is cut off and the furnace is cooled for 90 min and then sent to the forging process;

[0172] S7. Forging:

[0173] The electroslag ingots obtained in step S6 are subjected to ultra-high temperature diffusion control: the heating temperature is 1290 °C, and the holding time is 40 h for diffusion homogenization, and then it is upset and drawn in three directions of X, Y, and Z to open the billet, and finally it is drawn to the finished product size in one heat, the heating temperature is 1150 °C, and the deformation amount in the last heat is 50%. After forging, extreme cooling control is carried out, and it is water-cooled once to the large surface temperature of the billet of 280 °C;

[0174] S8. Isothermal normalizing treatment:

[0175] The forged material obtained in step S7 is subjected to isothermal normalizing: the blank is heated to 1000 °C and held for 34 h; after the holding is completed, it is air-cooled using a fog cannon fan, reheated to 740 °C after the large surface center is cooled to 500 °C, and held for 68 h. The above process is repeated 3 times. After the last holding is completed, the furnace is cooled to 300 °C and then taken out of the furnace for air-cooling.

[0176] S9. Ultra-fine grain treatment:

[0177] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020 °C with the furnace. After holding, extreme cooling control is adopted: direct water-cooling method, and the maximum return temperature of the large surface after cooling is 180 °C;

[0178] S10. Spheroidizing annealing:

[0179] The workpiece obtained in step S9 is placed in an annealing furnace, heated to 880 °C, held for 40 h, cooled to 740 °C after the holding is completed, held for 80 h for spheroidizing annealing, and after the holding is completed, the furnace is cooled to below 300 °C and then taken out of the furnace for air-cooling.

[0180] Example 5

[0181] A hot work die steel for high-ductility, high-uniformity, high-purity, and large-size die casting, by mass fraction, is basically composed of the elements shown in Tables 5-1 and 5-2:

[0182] Table 5-1 Chemical composition %

[0183] C Mn Si S P Cr Mo V Ni 0.34 0.40 0.97 0.001 0.007 5.10 1.50 0.62 0.40

[0184] Note: The rest is Fe

[0185] Table 5-2 Harmful elements % and residual gases (ppm)

[0186] Cu Ti Sn Pb H O N 0.02 0.0016 0.001 0.002 1.5 8 53

[0187] A preparation method of a hot work die steel for high-ductility, high-uniformity, high-purity, and large-size die casting, the preparation method is as follows:

[0188] S1. Electric furnace smelting:

[0189] According to the component content of the hot work die steel, blast furnace molten iron (mixing ratio 85%) and alloy materials are proportioned, CaO, CaF2, aluminum blocks, and dolomite are added, and melted and smelted in an electric furnace; after slag skimming, CaO, cleaning agent, and aluminum blocks are added and then tapped, and the tapping temperature is 1650 °C;

[0190] S2. Ladle furnace refining:

[0191] LF ladle refining furnace seat heating, using Fe-Si powder and Si-C slag surface diffusion deoxidation, adding an appropriate amount of CaO; refining time is 40 minutes, white slag time is 26 minutes; for alloy composition adjustment, metallic manganese, high-carbon ferrochrome, low-carbon ferrochrome, ferromolybdenum, ferrovanadium, etc. are selected.

[0192] S3. Vacuum refining furnace refining:

[0193] The vacuum refining furnace pumps step by step, the vacuum degree reaches 67 Pa, the vacuum holding time is 26 minutes, ensuring that the residual gas H ≤ 1.5 ppm. After the vacuum degassing is completed, sampling and analysis are carried out. After the composition is qualified, argon is blown until the ladle is lifted, and the soft blowing argon time is 24 min;

[0194] S4. Continuous casting of electrode billets:

[0195] Superheat is 30°C; continuous casting speed is 0.13 m / min, and three-stage electromagnetic stirring of the mold section, casting stream section, and continuous casting end section and two-stage water cooling are adopted; the surface temperature of the continuous casting billet is 760 - 800°C.

[0196] S5. Annealing of electrode billets:

[0197] The electrode billets continuously cast in step S4 are flame cut into segments and then annealed. The trolley furnace is preheated to 500°C for 3 h, slowly heated to 880°C for recrystallization annealing, the holding time is 40 h, and the furnace is cooled to 300°C and then taken out of the furnace.

[0198] S6. Electroslag remelting:

[0199] Using the electrode billets obtained in step S5, the surface is turned smooth by a lathe, and smelting is carried out using a four-component pre-melted slag of high-purity calcium fluoride, alumina, calcium oxide, and magnesium oxide (where the impurity silicon dioxide is 0.54%, ferrous oxide is 0.15%, and moisture is 0.005%). For the 15 t ingot type, constant melting speed control is adopted: the starting melting speed value at the steady state stage of electroslag smelting is 11.5 kg / min, the ending melting speed value is 9.0 kg / min, an electroslag ingot is obtained, and then the power is cut off and the furnace is cooled for 90 min and then sent to the forging process;

[0200] S7. Forging:

[0201] The electroslag ingots obtained in step S6 are subjected to ultra-high temperature diffusion control: the heating temperature is 1290°C, and diffusion homogenization is carried out by holding for 40 h, and then cross-rolling and upsetting are carried out in three directions of X, Y, and Z to open the billet. Finally, in the last heat, it is drawn to the finished product size, the heating temperature is 1150°C, and the deformation amount in the last heat is 52%. After forging, extreme cooling control is carried out, and it is directly water-cooled to the billet large surface temperature of 280°C at one time;

[0202] S8. Isothermal normalizing treatment:

[0203] The forged material obtained in step S7 is subjected to isothermal normalizing: the blank is heated to 1000 °C and held for 34 h; after holding, it is air-cooled using a fog cannon fan. After the large surface center cools to 500 °C, it is reheated to 740 °C and held for 68 h. The above process is repeated 3 times. After the last holding, the furnace is cooled to 300 °C and then taken out of the furnace for air-cooling.

[0204] S9. Ultra-fine grain treatment:

[0205] The workpiece obtained in step S8 is placed in a heating furnace and heated to 1020 °C along with the furnace. After holding, extreme cooling control is adopted: direct water-cooling method, and the maximum return temperature of the large surface after cooling is 215 °C;

[0206] S10. Spheroidizing annealing:

[0207] The workpiece obtained in step S9 is placed in an annealing furnace, heated to 880 °C, held for 44 h, and after holding, it is cooled to 740 °C and held for 88 h for spheroidizing annealing. After holding, the furnace is cooled to below 300 °C and then taken out of the furnace for air-cooling.

[0208] In order to verify the mechanical properties of the hot work die steel provided by the preparation method of the present invention, the inventor took 5 specimens of the die steel obtained in Examples 1 to 5 respectively for microscopic structure detection, as Figures 1 to 15 shown; the test data of performance detection are shown in Table 6 and Table 7:

[0209] Table 6

[0210]

[0211] Table 7

[0212]

[0213] It can be seen from this:

[0214] 1. It can be seen from the microscopic structures photographed in Examples 1 to 5 that the spheroidized annealing microstructure of this die steel is uniform, and fine spherical secondary carbides are uniformly distributed on the ferrite matrix. No network carbides precipitated along the grain boundaries, carbide blanks in some areas caused by local segregation, needle-shaped carbide precipitates with different orientations in the distribution arrangement, severe segregation, and carbide precipitation along the grain boundaries were observed. Rated according to the NADCA#207-2011 standard, the grade is AS2 - AS4. The uniform and fine spheroidized structure is beneficial to the dissolution and re-dispersion precipitation of carbides during the quenching and tempering processes of the die material in subsequent use, which not only reduces the performance differences in different parts of the die, but also achieves the purpose of improving the impact toughness;

[0215] 2. During the quenching process, most alloy elements of the mold material are dissolved into the matrix, and the Ni element is dissolved into the austenite to improve the impact toughness of the mold steel. At the same time, the carbides of Mo, Cr, and V are evenly distributed in the matrix, which can improve the strength, toughness, tempering stability, and wear resistance of the material. The mold steel material sample was tested for a single V-notch transverse impact energy ≥ 22J, and the average V-notch transverse impact performance ≥ 25J;

[0216] 3. From the microstructure photos taken in Examples 1 to 5, it can be concluded that there is no liquid precipitation carbide;

[0217] 4. From the microstructure photos taken in Examples 1 to 5, it can be concluded that there is no network carbide precipitation along the grain boundary, no carbide blanks in some areas caused by local segregation, the distribution and arrangement of precipitated carbides in needles with different orientations, severe segregation, and precipitation of carbides along the grains, etc.;

[0218] 5. From the detailed list of microscopic inclusions sampled and tested in Examples 1 to 5, it can be seen that only 0.5 grade D inclusions are contained, and the material purity is high;

[0219] 6. From the detailed table of austenite grain size results of samples taken and tested in Examples 1 to 5, it can be seen that the austenite grain size rating is 7.5-8.0, which exceeds the requirement of grade 7 or above for high-quality mold steel in the NADCA#207-2011 standard;

[0220] 7. It can be seen from the cross-sectional dimensions of the mold flat steel in Examples 1 to 5 that the maximum thickness is 800 mm and the maximum width is 1250 mm, which can well meet the production of large-scale integrated aluminum die-casting molds of different sizes.

[0221] In summary, the mold steel prepared by this preparation method has the characteristics of high toughness, high uniformity, high purity and large size.

Claims

1. A preparation method of a hot work die steel for large-size die casting with high toughness, high uniformity, high purity, characterized in that, the main steps are as follows: S1. Electric furnace smelting: Batch the blast furnace hot metal, sheared charge and ladle remaining steel according to the component content of the hot work die steel, where the weight ratio of the blast furnace hot metal is ≥80%, and add CaO, CaF 2 , aluminum blocks, and dolomite, and melt and smelt in an electric furnace; after slag skimming, add CaO, cleaning agent, and aluminum blocks and then tap the steel, and the tapping temperature is ≥1620°C; Among them, the component content of the hot work die steel is, by mass percentage, C 0.33% - 0.38%, Mn 0.30% - 0.50%, Si 0.80% - 1.10%, S ≤ 0.001%, P ≤ 0.008%, Cr 4.90% - 5.10%, Mo 1.30% - 1.60%, V 0.50% - 0.70%, Ni 0.25% - 0.45%, and the rest is Fe, and the residual gas content is, H ≤ 1.5 ppm, O ≤ 10 ppm, N ≤ 60 ppm; the residual harmful elements Cu ≤ 0.05%, Ti, Sn, Pb are all ≤ 0.002%; S2. Ladle furnace refining: The ladle of the LF ladle furnace is heated, and Fe-Si powder and Si-C powder are used for slag surface diffusion deoxidation, and an appropriate amount of CaO is added; the refining time ≥ 35 minutes, and the white slag time ≥ 20 minutes; S3. Vacuum refining furnace refining: The vacuum refining furnace pumps step by step, the vacuum degree reaches 67 Pa, the vacuum holding time ≥ 10 minutes, ensuring that the residual gas H ≤ 1.5 ppm. After the vacuum degassing is completed, sampling and analysis are carried out. After the composition is qualified, argon is blown until the ladle is lifted, and the soft blowing argon time is ≥ 15 min; S4. Continuous casting electrode billet: The superheat degree is 20 - 35 °C for the first furnace and 18 - 30 °C for the continuous casting furnace; the continuous casting speed is 0.12 - 0.14 m / min; S5. Electrode billet annealing: The electrode billet obtained by continuous casting in step S4 is flame cut into segments and then annealed. The trolley furnace is preheated ≥ 500 °C for ≥ 2 h, slowly heated to 860 - 880 °C for recrystallization annealing, the holding time ≥ 35 h, and the furnace is cooled to 300 °C and then taken out of the furnace; S6. Electroslag remelting: Using the electrode billet obtained in step S5, the surface is turned smooth by a lathe, and a four-component slag system of calcium fluoride, alumina, calcium oxide, and magnesium oxide is used for smelting to obtain an electroslag ingot, and then the electric furnace is cooled for 80 - 100 min and sent to the forging process; S7. Forging: The electroslag ingot obtained in step S6 is subjected to ultra-high temperature homogenization diffusion control. The heating temperature of ultra-high temperature homogenization is ≥ 1280 °C, and diffusion homogenization is carried out for 30 - 45 h, and then it is upset and drawn in three directions of X, Y, and Z to open the billet, and finally it is drawn to the finished size in one heat; S8. Isothermal normalizing treatment: The forged material obtained in step S7 is subjected to isothermal normalizing treatment; In step S8, the specific operation of isothermal normalizing treatment is: the billet is heated to 980 - 1030 °C, and the holding time ≥ 25 h; after the holding is completed, it is air-cooled. When the center of the large surface of the billet is cooled to 450 - 550 °C, it is reheated to 720 - 770 °C for holding, and the holding time ≥ 50 h; the above process is repeated ≥ 2 times; after the last holding is completed, the furnace is cooled to 300 °C and then taken out of the furnace and air-cooled; In step S8, a fog cannon fan is used for air-cooling after the holding is completed; S9. Ultra-fine grain treatment.

2. According to claim 1, a preparation method of a hot work die steel for large-size die casting with high toughness, high uniformity, high purity, It is characterized in that In step S2, one or several of metallic manganese, high-carbon ferrochrome, low-carbon ferrochrome, ferromolybdenum, ferrovanadium and ferronickel are selected for alloy composition adjustment.

3. The preparation method of a hot work die steel for high-toughness, high-uniformity, high-purity and large-size die casting according to claim 1, It is characterized in that In step S4, three-stage electromagnetic stirring of a mold section, a casting stream section and a continuous casting end section and two-stage water cooling are adopted; the surface temperature of the continuous casting billet is ≤800°C.

4. The preparation method of a hot work die steel for high-toughness, high-uniformity, high-purity and large-size die casting according to claim 1, It is characterized in that In step S6, for ingot molds of 15 tons to 25 tons, constant melting rate control is adopted: the starting melting rate value at the steady state stage of electroslag remelting is 10 - 13.5 kg / min, and the ending melting rate value is 8.0 - 10.0 kg / min.

5. The preparation method of a hot work die steel for high-toughness, high-uniformity, high-purity and large-size die casting according to claim 1, It is characterized in that In step S7, the heating temperature during forging is 1140 - 1170°C, and the deformation amount in the last heat is ≥50%; for post-forging ultimate cooling control, water-cooled directly to the core temperature of the billet ≤350°C.

6. The preparation method of a hot work die steel for high-toughness, high-uniformity, high-purity and large-size die casting according to claim 1, It is characterized in that In step S9, the specific steps of ultra-fine grain treatment are as follows; (A) Solution treatment: Place the workpiece obtained in step S8 into a heating furnace and heat it up with the furnace to 980 - 1030°C. After heat preservation, adopt ultimate cooling control: direct water-cooling method, and the maximum surface reversion temperature after cooling is 180 - 220°C; (B) Spheroidizing annealing: Place the workpiece obtained in step (A) into an annealing furnace, heat it to 860 - 880°C, keep it warm for 20 - 30 h, after the heat preservation ends, cool it down to 740 - 760°C, keep it warm for 40 - 60 h for spheroidizing annealing, and after the heat preservation ends, cool it in the furnace to below 300°C and take it out for air cooling.

7. A hot work die steel for high-toughness, high-uniformity, high-purity and large-size die casting prepared by the preparation method according to any one of claims 1 - 6.

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