High-purity and high-toughness plastic die steel and preparation method thereof

Abstract

The invention relates to high-purity and high-toughness plastic die steel and a preparation method thereof. The preparation method mainly comprises the following steps: S1, smelting in an electric furnace; s2, refining in a ladle refining furnace; s3, refining in a vacuum refining furnace; s4, continuously casting and casting billets, wherein the superheat degree of a first furnace is 20-35 DEG C, and the superheat degree of a continuous casting furnace is The continuous casting pulling speed is 0.13-0.20 m / min, and three-section electromagnetic stirring is adopted; s5, annealing the continuous casting billet; s6, forging is conducted; s7, after-forging waste heat normalizing is conducted, specifically, after-forging air cooling is conducted on the forging stock obtained in the step S6, and the large surface temperature ranges from 250 DEG C to 300 DEG C; s8, normalizing treatment is conducted; and S9, pre-hardening treatment is conducted. On the basis of common plastic die steel 1.2311, by adjusting chemical components and adding special smelting, forging and heat treatment processes, the prepared product has excellent purity, structure uniformity and mechanical property, and a die made of the material can better meet and adapt to the requirements of modern industrial production for high-end dies.

Description

Technical Field

[0001] The present invention relates to a die steel and a preparation method thereof, in particular to a high-purity and high-toughness plastic die steel and a preparation method thereof. Background Art

[0002] With the rapid development of China's plastic industry and the emergence of specialized die manufacturing factories, the demand for plastic product molding is increasing, and the quality requirements for plastic die steel are also getting higher and higher. At the same time, the development of China's plastic die steel shows that the price fluctuations of domestic and imported products have fallen into a polarized state. On the one hand, domestic low-end plastic die materials have fallen into a vicious price war. In the environment of rising raw material prices, the price of this product has caused huge economic pressure on the sales of enterprises, and enterprises are generally operating difficultly. On the other hand, medium and high-end products are seriously dependent on imports. Taking imported products such as German Grus 1.2738, Swedish Assab 718HH, and Japanese Daido Special Steel NAK80 and P20 as examples, their prices are 2-7 times that of domestic similar products, with rich profits. On the one hand, due to the relatively low market access threshold for low-end products, the product price has fallen into vicious competition due to the increase in intervening manufacturers, resulting in a continuous decline in enterprise profits. On the other hand, some high-end and high-profit die steel products have high technical thresholds and low enterprise brand awareness, and cannot be mass-produced and sold in China, resulting in the phenomenon that imported products occupy a large share of the domestic high-end die market.

[0003] Since traditional plastic die steel types can no longer meet the high-quality requirements such as complex structure, high dimensional accuracy, and high polishing. Therefore, it is imperative to develop new technologies, new processes, and new varieties of die steel. Summary of the Invention

[0004] The first object of the present invention is to provide a preparation method of a high-purity and high-toughness plastic die steel. Based on the common plastic die steel 1.2311, through the adjustment of chemical components and the addition of special smelting, forging, and heat treatment processes, the prepared product has excellent purity, tissue uniformity, and mechanical properties. The die made of this material can better meet the requirements of modern industrial production for high-end dies.

[0005] To achieve the above object of the invention, a preparation method of a high-purity and high-toughness plastic die steel mainly comprises the following steps:

[0006] S1. Electric furnace smelting:

[0007] The chemical component contents of the plastic mold steel, by mass percentage, are C 0.33% - 0.42%, Mn 1.20% - 1.60%, Si 0.20% - 0.40%, S ≤ 0.002%, P ≤ 0.010%, Cr 1.70% - 2.20%, Mo 0.20% - 0.40%, Ni 0.20% - 0.35%, the rest is Fe, and the residual gas contents are H ≤ 1.5 ppm, O ≤ 13 ppm, N ≤ 70 ppm; the residual harmful element Cu ≤ 0.05%, and Ti, Sn, Pb are all ≤ 0.002%;

[0008] S2, refining in a ladle furnace;

[0009] S3, refining in a vacuum refining furnace;

[0010] S4, continuous casting of continuous casting billets:

[0011] 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 0.13 - 0.20 m / min, and three-stage electromagnetic stirring is adopted;

[0012] S5, annealing of continuous casting billets:

[0013] Anneal the continuous casting billets after continuous casting in step S4;

[0014] S6, forging:

[0015] Forging the continuous casting billets obtained in step S5;

[0016] S7, normalizing with post-forging residual heat:

[0017] Air-cool the forged billets obtained in step S6, with the large surface temperature at 250 - 300 °C;

[0018] S8, normalizing treatment:

[0019] Place the workpieces obtained in step S7 into a heat treatment furnace for normalizing treatment;

[0020] S9, pre-hardening treatment:

[0021] Place the workpieces obtained in step S8 into a heating furnace for pre-hardening treatment.

[0022] Specifically, in step S1, charge the blast furnace hot metal, sheared furnace charge, and large ladle surplus steel according to the component contents of the plastic mold steel, where the proportion of blast furnace hot metal is ≥ 70%;

[0023] Add CaO, CaF2, aluminum blocks, and dolomite, and melt and smelt in an electric furnace;

[0024] After slag skimming, add CaO, cleaning agent, and aluminum blocks and then tap the steel, with the tapping temperature ≥ 1620 °C.

[0025] Specifically, in step S2, the ladle furnace seats the ladle for heating, and slag materials such as CaO, CaF2, and C-Si powder are added to reduce and adjust the slag system.

[0026] Specifically, in step S3, vacuum circulation refining is carried out using a vacuum circulation refining furnace. The molten steel treated in step S2 is poured into an integrated circulation vacuum tank and, in cooperation with a vacuum pump, the vacuum degree is ≤15 Pa; under vacuum conditions, the circulation flow rate is ≥100 t / min, the treatment time is ≥35 min, ensuring that the residual gases H ≤ 1.5 ppm and N ≤ 70 ppm. After vacuum degassing is completed, a sample is taken for analysis. After the composition is qualified, argon is blown until the ladle is lifted.

[0027] Furthermore, in step S3, the time for soft blowing of argon is 18 - 39 min, and the ladle temperature is 1550 - 1568 °C.

[0028] Specifically, in step S5, the annealing temperature is 640 - 680 °C, the holding time is 1 - 1.5 min / mm, and it is furnace-cooled to below 350 °C before discharging from the furnace.

[0029] Specifically, in step S6, the continuous casting billet obtained in step S5 is heated to 1230 - 1250 °C, held for 30 - 50 h for diffusion homogenization, then multi-directional forging in three directions of X, Y, and Z is carried out to complete the forging of the intermediate billet, and then it is returned to the furnace at 1180 - 1200 °C, held for 2 - 3 h, and drawn in the Z direction to the finished size.

[0030] Specifically, in step S8, the workpiece obtained in step S7 is placed in a heat treatment furnace and heated with the furnace to 870 - 890 °C. After holding for 7 - 15 h, it is pulled out of the trolley and air-cooled. The air-cooling on the trolley is to the center of the large surface at 250 - 300 °C. Then it is heated with the furnace to 860 - 880 °C. After holding for 7 - 15 h, it is pulled out of the trolley and air-cooled. The air-cooling on the trolley is to the center of the large surface at 250 - 300 °C. After being lifted off the furnace platform, it is air-cooled to room temperature.

[0031] Furthermore, in step S9, the workpiece obtained in step S8 is placed in a heating furnace and heated with the furnace to 880 - 900 °C. After holding, it is cooled by direct water cooling. The cooling end temperature is 150 - 200 °C; after cooling is completed, the workpiece is placed in an annealing furnace, heated to 530 - 540 °C, held for 15 - 30 h, furnace-cooled to 300 - 350 °C and then discharged from the furnace and air-cooled to room temperature. Then it is reheated to 535 - 545 °C, held for 15 - 30 h, furnace-cooled to 280 - 330 °C and then discharged from the furnace and air-cooled to room temperature.

[0032] Another object of the present invention is to provide a highly pure and highly tough plastic mold steel prepared by the above-mentioned preparation method. The chemical component contents are, by mass percentage, C 0.33% - 0.42%, Mn 1.20% - 1.60%, Si 0.20% - 0.40%, S ≤ 0.002%, P ≤ 0.010%, Cr 1.70% - 2.20%, Mo 0.20% - 0.40%, Ni 0.20% - 0.35%, the balance being Fe, and the residual gas contents are H ≤ 1.5 ppm, O ≤ 13 ppm, N ≤ 70 ppm; the residual harmful element Cu ≤ 0.05%, and Ti, Sn, Pb are all ≤ 0.002%.

[0033] A highly pure and highly tough plastic mold steel and its preparation method according to the present invention have the following advantages compared with the prior art:

[0034] (1) By using a high proportion of blast furnace hot metal (≥70%) + selected high-quality scrap steel, alloy materials and other auxiliary materials, and supplemented by improved and optimized production processes (including electric furnace smelting process, LF+RH refining process, protective atmosphere casting process), the purity of the electrode blank is greatly improved, where P ≤ 0.008%, S ≤ 0.002%, the residual gas contents N ≤ 90 ppm, H ≤ 1.5 ppm, O ≤ 13 ppm, and the inclusion grade of A / B / C / D ≤ 0.5 level;

[0035] The addition of blast furnace hot metal reduces the proportion of scrap steel at the same time, which can reduce the impurity elements remaining in the scrap steel from entering the steel;

[0036] The addition of selected low-nitrogen ferrochromium can reduce the total nitrogen content in the molten steel and reduce the formation of nitrides;

[0037] The use of a vacuum circulation refining furnace enables the molten steel to fully contact the liquid refining slag during the circulation process, adsorb the inclusions in the molten steel by the interfacial tension between the slag droplets and the molten steel, and at the same time, the oxides (such as CaO, Al2O3) in the slag droplets react with the non-metallic inclusions (such as SiO2, MnO2) in the molten steel to form stable compounds. The size radius of the compounds increases, making it easy to float and be captured by the slag pool for removal.

[0038] (2) During the continuous casting process, the application of three-stage electromagnetic stirring can reduce the temperature gradient at the solidification front and reduce the aggregation of inclusions by improving the molten steel flow and heat transfer process; at the same time, it promotes the floating of inclusions and the discharge of bubbles and continuously updates the steel slag interface, which is beneficial to the protective slag absorbing inclusions. The purpose of removing inclusions and reducing the inclusion size is achieved.

[0039] (3) Through high-temperature diffusion at ultra-high temperature for a long time, carbon and other alloying elements are effectively dissolved and diffused, significantly improving the compositional segregation of continuous casting billets; under long-term high-temperature homogenization, the atomic thermal motion of carbon and alloying elements in the steel intensifies, accelerating the diffusion of elements from high-concentration regions to low-concentration regions, achieving the purpose of reducing banded segregation; the uniformity of carbon and alloying elements enables the austenite recrystallization and microstructure phase transformation in each microscopic region inside the forged material to tend to be consistent during the heat treatment process, obtaining uniform austenite grains and microstructure uniformity; the uniformity of carbon and alloying elements makes the types of alloy carbides in each microscopic region inside the forged material similar, resulting in the uniformity of macroscopic hardness.

[0040] (4) Through stress-strain simulation during forging using the thermal simulation software Simufact-Forming, a reasonable multi-directional deformation process is formulated, and the as-cast dendritic structure is fully broken and compacted, effectively improving banded segregation and anisotropy.

[0041] (5) Through air-cooling after forging with residual heat normalizing, corresponding cooling processes are formulated for billets of different specifications and sizes using the thermal simulation software Deform-HT, ensuring that the core temperature of the billet is 500 - 650 °C, fully suppressing the precipitation of network carbide and the growth of austenite grains. At the same time, a balanced microstructure is obtained, reducing forging microstructure heredity and providing good basic conditions for subsequent ultra-refinement.

[0042] (6) Through normalizing treatment: After solution treatment, furnace cooling is carried out on the trolley. Corresponding cooling processes are formulated for billets of different specifications and sizes using the thermal simulation software Deform-HT, controlling the cooling rate, reducing the content of free ferrite, and increasing the pearlite microstructure at the same time. The purpose of refining austenite grains is achieved through two repetitions.

[0043] (7) Through adjustment treatment: After solution treatment, direct water cooling is carried out. Corresponding cooling processes are formulated for billets of different specifications and sizes using the thermal simulation software Deform-HT, controlling the cooling rate, increasing the transformation amount of martensite microstructure, and reducing the content of free ferrite. Through two-step gradient temperature tempering, the microstructure and thermal stress after quenching of the billet are removed, and the hardness uniformity inside and outside the billet is ensured at the same time. Description of the Drawings

[0044] Figure 1 Microstructure diagram of the die steel obtained in Example 1;

[0045] Figure 2 Austenite grain size diagram of the die steel obtained in Example 1;

[0046] Figure 3 Microstructure diagram of the die steel obtained in Example 2;

[0047] Figure 4 Austenite grain size diagram of the die steel obtained in Example 2;

[0048] Figure 5 The metallographic structure diagram of the die steel obtained in Example 3;

[0049] Figure 6 The austenite grain size diagram of the die steel obtained in Example 3;

[0050] Figure 7 The metallographic structure diagram of the die steel obtained in Example 4;

[0051] Figure 8 The austenite grain size diagram of the die steel obtained in Example 4;

[0052] Figure 9 The metallographic structure diagram of the die steel obtained in Example 5;

[0053] Figure 10 The austenite grain size diagram of the die steel obtained in Example 5. Detailed implementation manners

[0054] The following further describes the present invention in conjunction with the accompanying drawings and specific embodiments.

[0055] Example 1

[0056] A high-purity and high-toughness plastic mold steel, by mass percentage, is basically composed of the elements in Table 1 as follows:

[0057] Table 1

[0058]

[0059] Its preparation method mainly includes the following steps:

[0060] S1. Electric furnace smelting:

[0061] According to the component content of the plastic mold steel, charge blast furnace hot metal (mixing ratio 80%), sheared furnace charge and large package surplus steel, add CaO, CaF2, aluminum blocks, and dolomite, and melt and smelt in an electric furnace; the molten steel temperature is 1640°C for oxidation slag removal. After slag removal, add alloy materials, lime and fluorite to adjust the chemical composition and dephosphorize. The tapping temperature is 1630°C. During the tapping process, add aluminum wire at a rate of 1.0 kg / t for deoxidation.

[0062] S2. Ladle furnace refining:

[0063] After the molten steel is smelted in the electric furnace, it is transferred into the ladle, hoisted to the refining furnace seat, and slag materials such as CaO, CaF2, and C-Si powder are added to reduce and produce white slag. Alloy materials are added according to the actual composition to complete the operations of composition fine-tuning, desulfurization, and deoxidation. After the operation is completed, the white slag is removed, and then it is reheated and new slag materials are added to produce white slag again to further remove inclusions in the steel. During the process of producing white slag again, aluminum wire and calcium wire are added for combined deoxidation. The dosage of both aluminum wire and calcium wire is added at 1.0 kg / t.

[0064] S3. Vacuum circulating refining furnace refining:

[0065] The molten steel treated in step S2 is poured into the integrated circulating vacuum tank and refined by a vacuum circulating refining furnace, cooperating with an advanced vacuum pump, with a vacuum degree of 12 Pa. Under vacuum conditions, the circulation flow rate is 120 t / min, and the treatment time is 37 minutes to ensure that the residual gases H ≤ 1.0 ppm and N ≤ 70 ppm. After vacuum degassing, a sample is taken for analysis. After the composition is qualified, argon is blown until the ladle is lifted, and the soft blowing time of argon is 25 min, and the ladle temperature is 1560 °C.

[0066] S4. Continuous casting of continuous casting billets:

[0067] The superheat degree is 30 °C for the first furnace and 25 °C for the continuous casting furnace; the continuous casting drawing speed is 0.16 m / min, and three-stage electromagnetic stirring is adopted.

[0068] S5. Annealing of continuous casting billets:

[0069] The continuous casting billets after continuous casting in step S4 are annealed at an annealing temperature of 850 °C for a holding time of 1.5 min / mm, and then cooled in the furnace to 340 °C and taken out of the furnace.

[0070] S6. Forging:

[0071] The continuous casting billets obtained in step S5 are heated to 1240 °C and held for 40 h for diffusion homogenization, and then the intermediate billets are forged by multi-directional forging. Then they are returned to the furnace at 1180 °C for a holding time of 3 h, and drawn in the Z direction to the finished product size.

[0072] S7. Normalizing after forging with after-forging waste heat

[0073] The forged billets obtained in step S6 are air-cooled after forging, and the large surface temperature is 290 °C.

[0074] S8. Normalizing treatment:

[0075] The workpieces obtained in step S7 are placed in the heat treatment furnace and heated to 880 °C with the furnace, held for 10 h, then pulled out of the trolley and air-cooled until the large surface center temperature reaches 275 °C. Then they are heated to 870 °C with the furnace, held for 10 h, pulled out of the trolley and air-cooled until the large surface center temperature reaches 250 °C, and then air-cooled to room temperature after being lifted off the furnace platform.

[0076] S9. Pre-hardening treatment:

[0077] Place the workpiece obtained in step S8 into a heating furnace and heat it up to 890 °C along with the furnace. After heat preservation, cool it by direct water cooling. The end temperature of cooling is 200 °C. After the cooling is completed, place the workpiece into an annealing furnace, heat it to 530 °C, keep it warm for 20 h, cool it in the furnace to 330 °C and then take it out of the furnace and air-cool it to room temperature. Then reheat it to 540 °C, keep it warm for 30 h, cool it in the furnace to 300 °C and then take it out of the furnace and air-cool it to room temperature.

[0078] Example 2

[0079] A high-purity and high-toughness plastic mold steel, by mass percentage, is basically composed of the elements shown in Table 2:

[0080] Table 2

[0081]

[0082] Its preparation method mainly includes the following steps:

[0083] S1. Electric furnace smelting:

[0084] According to the component content of the plastic mold steel, charge blast furnace hot metal (proportion 80%), sheared furnace charge and ladle residue steel, add CaO, CaF2, aluminum blocks and dolomite, and melt and smelt in an electric furnace. The molten steel temperature is 1640 °C for oxidation and slag skimming. After slag skimming, add alloy materials, lime and fluorite to adjust the chemical composition and dephosphorize. The tapping temperature is 1630 °C. During the tapping process, add aluminum wire at a rate of 1.0 kg / t for deoxidation.

[0085] S2. Ladle furnace refining:

[0086] After the electric furnace smelting, the molten steel is transferred to a ladle, lifted to the refining furnace seat, and add slag materials CaO, CaF2, C-Si powder to reduce and make a good white slag. Add alloy materials according to the actual composition to complete the fine adjustment of composition, desulfurization and deoxidation operations. After the operation is completed, remove the white slag, then reheat and add new slag materials for secondary white slag making to further remove inclusions in the steel. During the secondary white slag making process, add aluminum wire and calcium wire for composite deoxidation. The dosage of aluminum wire and calcium wire is both added at a rate of 1.0 kg / t.

[0087] S3. Vacuum circulation refining furnace refining:

[0088] Refined by a vacuum circulating refining furnace, the molten steel after being processed in step S2 is poured into an integrated circulating vacuum tank, and with the cooperation of an advanced vacuum pump, the vacuum degree is 12 Pa; under vacuum conditions, the circulation flow rate is 120 t / min, the treatment time is 37 minutes, ensuring that the residual gases H ≤ 1.0 ppm and N ≤ 70 ppm. After vacuum degassing, sampling and analysis are carried out. After the composition is qualified, argon is blown into the ladle, and the soft blowing time of argon is 25 min, and the ladle temperature is 1560 °C.

[0089] S4. Continuous casting of continuous casting billets:

[0090] The superheat degree is 30 °C for the first furnace and 25 °C for the continuous casting furnace; the continuous casting drawing speed is 0.16 m / min, and three-stage electromagnetic stirring is adopted.

[0091] S5. Annealing of continuous casting billets:

[0092] The continuous casting billets after continuous casting in step S4 are annealed, the annealing temperature is 850 °C, the holding time is 1.5 min / mm, and the furnace is cooled to 340 °C and then taken out of the furnace.

[0093] S6. Forging:

[0094] The continuous casting billets obtained in step S5 are heated to 1240 °C, held for 40 h for diffusion homogenization, then the intermediate billets are forged by multi-directional forging, and then returned to the furnace at 1180 °C, held for 3 h, and drawn in the Z direction to the finished product size.

[0095] S7. Normalizing after forging

[0096] The forged billets obtained in step S6 are air-cooled after forging, and the large surface temperature is 280 °C.

[0097] S8. Normalizing treatment:

[0098] The workpieces obtained in step S7 are placed in a heat treatment furnace and heated to 880 °C with the furnace, held for 10 h and then pulled out of the trolley and air-cooled. The air-cooling on the trolley reaches 285 °C at the center of the large surface. Then it is heated to 870 °C with the furnace, held for 10 h and then pulled out of the trolley and air-cooled. The air-cooling on the trolley reaches 270 °C at the center of the large surface, and then it is lifted off the furnace platform and air-cooled to room temperature.

[0099] S9. Pre-hardening treatment:

[0100] The workpieces obtained in step S8 are placed in a heating furnace and heated to 890 °C with the furnace. After holding, direct water cooling is used for cooling, and the cooling end temperature is 190 °C; after cooling, the workpieces are placed in an annealing furnace, heated to 530 °C, held for 20 h, cooled in the furnace to 330 °C and then taken out of the furnace and air-cooled to room temperature. Then it is reheated to 540 °C, held for 30 h, cooled in the furnace to 300 °C and then taken out of the furnace and air-cooled to room temperature.

[0101] Example 3

[0102] A high-purity and high-toughness plastic mold steel, by mass percentage, is basically composed of the elements in Table 3 as follows:

[0103] Table 3

[0104]

[0105] Its preparation method mainly includes the following steps:

[0106] S1. Electric furnace smelting:

[0107] Charge the blast furnace hot metal (proportion 80%), sheared furnace charge and ladle residue steel according to the component content of the plastic mold steel, add CaO, CaF2, aluminum blocks, and dolomite, and melt and smelt in the electric furnace; the molten steel temperature is 1640°C for oxidation slag skimming. After slag skimming, add alloy materials, lime, and fluorite to adjust the chemical composition and dephosphorize. The tapping temperature is 1630°C. During the tapping process, add aluminum wire at a rate of 1.0 kg / t for deoxidation.

[0108] S2. Ladle furnace refining:

[0109] After the electric furnace smelting, the molten steel is transferred to the ladle, hoisted to the refining furnace seat, and add slag materials CaO, CaF2, and C-Si powder to reduce and make a good white slag. Add alloy materials according to the actual composition to complete the fine adjustment of composition, desulfurization, and deoxidation operations; after the operation is completed, remove the white slag, then reheat and add new slag materials for secondary white slag making to further remove inclusions in the steel; during the secondary white slag making process, add aluminum wire and calcium wire for composite deoxidation; the dosages of aluminum wire and calcium wire are both added at a rate of 1.0 kg / t.

[0110] S3. Vacuum circulation refining furnace refining:

[0111] Refine with a vacuum circulation refining furnace. Pour the molten steel treated in step S2 into an integrated circulation vacuum tank, cooperate with an advanced vacuum pump, and the vacuum degree is 12 Pa; under vacuum conditions, the circulation flow rate is 120 t / min, and the treatment time is 37 minutes to ensure that the residual gas H ≤ 1.0 ppm and N ≤ 70 ppm. After vacuum degassing, take a sample for analysis. After the composition is qualified, blow argon to the ladle for lifting. The soft blowing argon time is 25 min, and the ladle temperature is 1560°C.

[0112] S4. Continuous casting of continuous casting billets:

[0113] The superheat is 30°C for the first furnace and 25°C for the continuous casting furnace; the continuous casting drawing speed is 0.16 m / min, and three-stage electromagnetic stirring is adopted.

[0114] S5. Annealing of continuous casting billets:

[0115] Anneal the continuous casting billet after continuous casting in step S4, with an annealing temperature of 850 °C, a holding time of 1.5 min / mm, and furnace cool to 340 °C before discharging from the furnace.

[0116] S6. Forging:

[0117] Heat the continuous casting billet obtained in step S5 to 1240 °C, hold for 40 h for diffusion homogenization, then complete the forging of the intermediate billet through multi-directional forging, and then return to the furnace at 1180 °C, hold for 3 h, and perform upsetting in the Z direction to the finished size.

[0118] S7. Normalizing after forging with residual heat

[0119] Air-cool the forged billet obtained in step S6 after forging, with the large surface temperature of 280 °C.

[0120] S8. Normalizing treatment:

[0121] Place the workpiece obtained in step S7 into the heat treatment furnace and heat it up to 880 °C along with the furnace, hold for 10 h and then pull it out of the trolley for air cooling until the large surface center temperature reaches 280 °C. Then heat it up to 870 °C along with the furnace, hold for 10 h and then pull it out of the trolley for air cooling until the large surface center temperature reaches 270 °C, and then lift it off the furnace platform and air cool it to room temperature.

[0122] S9. Pre-hardening treatment:

[0123] Place the workpiece obtained in step S8 into the heating furnace and heat it up to 890 °C along with the furnace. After holding, cool it by direct water cooling, and the cooling end temperature is 190 °C; after cooling, place the workpiece into the annealing furnace, heat it to 530 °C, hold for 20 h, furnace cool to 330 °C and then discharge from the furnace and air cool to room temperature. Then reheat it to 540 °C, hold for 30 h, furnace cool to 300 °C and then discharge from the furnace and air cool to room temperature.

[0124] Example 4

[0125] A high-purity and high-toughness plastic mold steel, by mass percentage, is basically composed of the elements shown in Table 4:

[0126] Table 4

[0127]

[0128] Its preparation method mainly includes the following steps:

[0129] S1. Electric furnace smelting:

[0130] Batch the blast furnace molten iron (proportion 80%), sheared furnace charge and ladle residue steel according to the component content of plastic mold steel, add CaO, CaF2, aluminum blocks and dolomite, and melt and smelt in an electric furnace; oxidize and slag off at a molten steel temperature of 1640 °C. After slagging off, add alloy materials, lime and fluorite to adjust the chemical composition and dephosphorize. The tapping temperature is 1630 °C. During tapping, add aluminum wire at a rate of 1.0 kg / t for deoxidation.

[0131] S2. Refining in a ladle furnace:

[0132] After the molten steel from the electric furnace smelting is transferred to the ladle and hoisted to the ladle furnace seat, add slag materials CaO, CaF2, C-Si powder to reduce and make a good white slag. Add alloy materials according to the actual composition to complete the fine adjustment of composition, desulfurization and deoxidation operations; after the operation is completed, remove the white slag, then reheat and add new slag materials to make white slag again to further remove inclusions in the steel; during the process of making white slag again, add aluminum wire and calcium wire for composite deoxidation; the dosage of both aluminum wire and calcium wire is added at a rate of 1.0 kg / t.

[0133] S3. Refining in a vacuum circulating refining furnace:

[0134] Use a vacuum circulating refining furnace for refining. Pour the molten steel treated in step S2 into an integrated circulating vacuum tank, cooperate with an advanced vacuum pump, and the vacuum degree is 12 Pa; under vacuum conditions, the circulation flow rate is 120 t / min, and the treatment time is 37 minutes to ensure that the residual gas H ≤ 1.0 ppm and N ≤ 70 ppm. After vacuum degassing, take samples for analysis. After the composition is qualified, blow argon until ladling, and the soft blowing argon time is 25 min, and the ladling temperature is 1560 °C.

[0135] S4. Continuous casting of continuous casting billets:

[0136] The superheat degree is 30 °C for the first furnace and 25 °C for the continuous casting furnace; the continuous casting drawing speed is 0.16 m / min, and three-stage electromagnetic stirring is adopted.

[0137] S5. Annealing of continuous casting billets:

[0138] Anneal the continuous casting billets after continuous casting in step S4. The annealing temperature is 850 °C, the holding time is 1.5 min / mm, and cool in the furnace to 340 °C and then take out of the furnace.

[0139] S6. Forging:

[0140] Heat the continuous casting billets obtained in step S5 to 1240 °C, hold for 40 h for diffusion homogenization, then complete the forging of the intermediate billets through multi-directional forging, and then return to the furnace at 1180 °C, hold for 3 h, and perform upsetting in the Z direction to the finished product size.

[0141] S7. Normalizing after forging with residual heat

[0142] The forged billet obtained in step S6 is air-cooled after forging, and the temperature of the large surface is 290 °C.

[0143] S8. Normalizing treatment:

[0144] The workpiece obtained in step S7 is placed in a heat treatment furnace and heated to 880 °C along with the furnace. After holding for 10 h, it is pulled out of the trolley and air-cooled. The workpiece is air-cooled on the trolley until the temperature at the center of the large surface reaches 280 °C. Then it is heated to 870 °C along with the furnace. After holding for 10 h, it is pulled out of the trolley and air-cooled. The workpiece is air-cooled on the trolley until the temperature at the center of the large surface reaches 270 °C. After being lifted off the furnace platform, it is air-cooled to room temperature.

[0145] S9. Pre-hardening treatment:

[0146] The workpiece obtained in step S8 is placed in a heating furnace and heated to 890 °C along with the furnace. After holding, it is cooled by direct water cooling. The end temperature of cooling is 180 °C. After the cooling is completed, the workpiece is placed in an annealing furnace, heated to 530 °C, held for 20 h, cooled in the furnace to 330 °C and then taken out of the furnace and air-cooled to room temperature. Then it is reheated to 540 °C, held for 30 h, cooled in the furnace to 300 °C and then taken out of the furnace and air-cooled to room temperature.

[0147] Example 5

[0148] A high-purity and high-toughness plastic mold steel, by mass fraction, is basically composed of the elements shown in Table 5:

[0149] Table 5

[0150]

[0151] Its preparation method mainly includes the following steps:

[0152] S1. Electric furnace smelting:

[0153] According to the component content of the plastic mold steel, the burdening of blast furnace hot metal (proportion 80%), sheared furnace charge and ladle remaining steel is carried out, and CaO, CaF2, aluminum blocks and dolomite are added and melted and smelted in an electric furnace. The molten steel temperature is 1640 °C for oxidation and slag skimming. After slag skimming, alloy materials, lime and fluorite are added to adjust the chemical composition and dephosphorize. The tapping temperature is 1630 °C. During the tapping process, aluminum wire is added at a rate of 1.0 kg / t for deoxidation.

[0154] S2. Ladle furnace refining:

[0155] After the molten steel is smelted in the electric furnace, it is transferred into a ladle, hoisted onto the refining furnace seat, and slag materials such as CaO, CaF2, and C-Si powder are added to reduce and produce a white slag. Alloy materials are added according to the actual composition to complete the operations of fine composition adjustment, desulfurization, and deoxidation. After the operation is completed, the white slag is removed, and then it is reheated and new slag materials are added to produce a white slag for the second time to further remove inclusions in the steel. During the process of producing the white slag for the second time, aluminum wire and calcium wire are added for combined deoxidation. The dosage of both aluminum wire and calcium wire is added at 1.0 kg / t.

[0156] S3. Vacuum circulation refining furnace refining:

[0157] Refining is carried out using a vacuum circulation refining furnace. The molten steel treated in step S2 is poured into an integrated circulating vacuum tank, and with the cooperation of an advanced vacuum pump, the vacuum degree is 12 Pa. Under vacuum conditions, the circulation flow rate is 120 t / min, and the treatment time is 37 minutes to ensure that the residual gases H ≤ 1.0 ppm and N ≤ 70 ppm. After vacuum degassing, a sample is taken for analysis. After the composition is qualified, argon is blown until the ladle is lifted, and the soft blowing time of argon is 25 min, and the ladle temperature is 1560 °C.

[0158] S4. Continuous casting of continuous casting billets:

[0159] The superheat is 30 °C for the first furnace and 25 °C for the continuous casting furnace; the continuous casting drawing speed is 0.16 m / min, and three-stage electromagnetic stirring is adopted.

[0160] S5. Annealing of continuous casting billets:

[0161] The continuous casting billets after continuous casting in step S4 are annealed. The annealing temperature is 850 °C, the holding time is 1.5 min / mm, and it is cooled in the furnace to 340 °C and then taken out of the furnace.

[0162] S6. Forging:

[0163] The continuous casting billets obtained in step S5 are heated to 1240 °C and held for 40 h for diffusion homogenization, and then the intermediate billets are forged by multi-directional forging. Then they are returned to the furnace at 1180 °C and held for 3 h, and are drawn in the Z direction to the finished product size.

[0164] S7. Normalizing after forging with residual heat

[0165] The forged billets obtained in step S6 are air-cooled after forging, and the large surface temperature is 270 °C.

[0166] S8. Normalizing treatment:

[0167] The workpieces obtained in step S7 are placed in a heat treatment furnace and heated to 880 °C along with the furnace, held for 10 h and then pulled out of the trolley for air cooling. The air cooling on the trolley is to the large surface center at 290 °C. Then it is heated to 870 °C along with the furnace, held for 10 h and then pulled out of the trolley for air cooling. The air cooling on the trolley is to the large surface center at 270 °C, and then it is lifted off the furnace platform and air-cooled to room temperature.

[0168] S9. Pre-hardening treatment:

[0169] Place the workpiece obtained in step S8 into a heating furnace and heat it up to 890 °C with the furnace. After holding for a certain time, cool it by direct water cooling, and the end temperature of cooling is 190 °C. After the cooling is completed, place the workpiece into an annealing furnace, heat it to 530 °C, hold for 20 h, cool it in the furnace to 330 °C, and then take it out of the furnace and air-cool it to room temperature. Then reheat it to 540 °C, hold for 30 h, cool it in the furnace to 300 °C, and then take it out of the furnace and air-cool it to room temperature.

[0170] In order to verify the mechanical properties of the plastic mold steel provided by the preparation method of the present invention, the inventors respectively took samples of the mold steels obtained in Examples 1 to 5 for mechanical property tests (the test results are shown in Table 6), purity detection (the test results are shown in Table 7), and metallographic structure analysis (such as Figures 1 to 10 ).

[0171] Table 6

[0172]

[0173] Table 7

[0174]

[0175] Thus, it can be seen that

[0176] 1. As Figures 1 - 10 shown, the pre-hardened microstructure of the mold material is uniform, tempered sorbite + a very small amount of free ferrite, and the rating according to the GB / T 11354 standard is Grade 1.

[0177] 2. As shown in Table 6, the solidification segregation of the continuous casting billet is reduced through processes such as the pouring temperature, drawing speed, and electromagnetic stirring of the continuous casting billet; the homogenization of alloying elements is achieved through the implementation of long-time high-temperature homogenization and multi-directional forging processes, so that the hardness uniformity of the cross-section after the material is quenched and tempered is improved to within ±1 HRC.

[0178] 3. As shown in Table 7, the purity of the hot metal in the electric furnace smelting is ensured by matching a high proportion of blast furnace hot metal, high-quality scrap steel, alloys, etc. At the same time, with the cooperation of an integrated vacuum circulation refining furnace, the molten steel and the refining slag are fully contacted and the power for the inclusions to float up sufficiently is given, so that the inclusions in the molten steel float up sufficiently and are floated up by the refining slag, achieving the purpose of obtaining pure molten steel. After forging the finished product, it is detected according to the GB / T 10561 Grade A standard, and the evaluation level ≤ 0.5 grade.

[0179] Obtain an inclusion level below grade 0.5 through purity control; obtain a tempered sorbite structure with high toughness through the regulation of the structure during the quenching and tempering process; obtain a fine austenite grain size through the implementation of multi-directional forging and normalizing processes (the grain size rating ≥ grade 8.0 as detected according to the GB / T 6394 standard); achieve the purpose of improving the impact toughness through the combined action of reducing microcrack initiation points, strengthening and toughening the structure, and strengthening and toughening the grain boundaries, with the average value of the notch-free impact energy ≥ 250 J.

[0180] 4. From the microscopic structure photos taken in Examples 1 - 10, the structure is uniform: tempered sorbite + a very small amount of free ferrite.

Claims

1. A preparation method of a high-purity and high-toughness plastic mold steel, characterized in that, The main steps are as follows: S1. Electric furnace smelting: For the chemical component content of plastic mold steel, by mass percentage, C is 0.33% - 0.42%, Mn is 1.20% - 1.60%, Si is 0.20% - 0.40%, S ≤ 0.002%, P ≤ 0.010%, Cr is 1.70% - 2.20%, Mo is 0.20% - 0.40%, Ni is 0.20% - 0.35%, the rest is Fe, and the residual gas content is H ≤ 1.5 ppm, O ≤ 13 ppm, N ≤ 70 ppm; the residual harmful element Cu ≤ 0.05%, and Ti, Sn, Pb are all ≤ 0.002%; S2. Ladle furnace refining; S3. Vacuum refining furnace refining; S4. Continuous casting of continuous casting billets: The superheat degree is 20 - 35°C for the first furnace and 18 - 30°C for continuous casting furnaces; the continuous casting drawing speed is 0.13 - 0.20 m / min, and three-stage electromagnetic stirring is adopted; S5. Annealing of continuous casting billets: Anneal the continuous casting billets after continuous casting in step S4; S6. Forging: Forging the continuous casting billets obtained in step S5; S7. Normalizing with post-forging residual heat: Air-cool the forged billets obtained in step S6, with the large surface temperature at 250 - 300°C; S8. Normalizing treatment: Place the workpieces obtained in step S7 into a heat treatment furnace for normalizing treatment; S9. Pre-hardening treatment: Place the workpieces obtained in step S8 into a heating furnace for pre-hardening treatment.

2. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 1, characterized in that, In step S1, proportion the blast furnace hot metal, sheared furnace charge and ladle remaining steel according to the component content of plastic mold steel, where the proportion of blast furnace hot metal ≥ 70%; 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 ≥ 1620°C.

3. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 1, characterized in that, In step S2, seat and heat the ladle furnace, and add slag materials CaO, CaF2, and C-Si powder to reduce and adjust the slag system.

4. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 1, characterized in that, In step S3, use a vacuum circulating refining furnace for refining. Pour the molten steel treated in step S2 into an integrated circulating vacuum tank, and cooperate with a vacuum pump, with the vacuum degree ≤ 15 Pa; under vacuum conditions, the circulation flow rate ≥ 100 t / min, the treatment time ≥ 35 min, ensure that the residual gas H ≤ 1.5 ppm, N ≤ 70 ppm. After vacuum degassing, take samples for analysis. After the composition is qualified, blow argon until ladling.

5. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 4, characterized in that, In step S3, the soft blowing argon time is 18 - 39 min, and the ladling temperature is 1550 - 1568°C.

6. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 1, characterized in that, In step S5, the annealing temperature is 640 - 680°C, the holding time is 1 - 1.5 min / mm, and furnace cool to below 350°C and then take out of the furnace.

7. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 1, characterized in that, In step S6, heat the continuous casting billets obtained in step S5 to 1230 - 1250°C, hold for 30 - 50 h for diffusion homogenization, then perform multi-directional forging in three directions of X, Y, and Z to complete the forging of the intermediate billet, and then return to the furnace at 1180 - 1200°C, hold for 2 - 3 h, and perform drawing in the Z direction to the finished size.

8. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 1, characterized in that, In step S8, the workpiece obtained in step S7 is placed in a heat treatment furnace and heated up to 870 - 890 °C along with the furnace. After holding for 7 - 15 h, it is taken out of the trolley and air-cooled. The air-cooling on the trolley is carried out until the center of the large surface reaches 250 - 300 °C. Then it is heated up to 860 - 880 °C along with the furnace, held for 7 - 15 h, taken out of the trolley and air-cooled. The air-cooling on the trolley is carried out until the center of the large surface reaches 250 - 300 °C. After being lifted off the furnace platform, it is air-cooled to room temperature.

9. The preparation method of a high-purity and high-toughness plastic mold steel according to claim 1, characterized in that, In step S9, the workpiece obtained in step S8 is placed in a heating furnace and heated up to 880 - 900 °C along with the furnace. After holding, it is cooled by direct water cooling. The end temperature of cooling is 150 - 200 °C. After the cooling is completed, the workpiece is placed in an annealing furnace, heated to 530 - 540 °C, held for 15 - 30 h, furnace-cooled to 300 - 350 °C and then taken out of the furnace and air-cooled to room temperature. Then it is reheated to 535 - 545 °C, held for 15 - 30 h, furnace-cooled to 280 - 330 °C and then taken out of the furnace and air-cooled to room temperature.

10. A highly pure and highly ductile plastic mold steel prepared by the preparation method according to any one of claims 1 to 5, characterized in that, The chemical component content of the hot work die steel, by mass percentage, is C 0.33% - 0.42%, Mn 1.20% - 1.60%, Si 0.20% - 0.40%, S ≤ 0.002%, P ≤ 0.010%, Cr 1.70% - 2.20%, Mo 0.20% - 0.40%, Ni 0.20% - 0.35%, and the rest is Fe. And the residual gas content is H ≤ 1.5 ppm, O ≤ 13 ppm, N ≤ 70 ppm; the residual harmful element Cu ≤ 0.05%, and Ti, Sn, Pb are all ≤ 0.002%.

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