High-hardness and high-toughness MoCr alloy cast iron for automobile drawing die and preparation process of high-hardness and high-toughness MoCr alloy cast iron

Abstract

The invention discloses high-hardness and high-toughness MoCr alloy cast iron for an automobile drawing die and a preparation process thereof.The alloy cast iron comprises, by mass, 3.1%-3.3% of C, 1.6%-1.8% of Si, 0.7%-0.9% of Mn, 0.8%-1.0% of Cr, 0.3%-0.4% of Mo, 0.2%-0.3% of Cu, 0.1%-0.2% of V, 0.05%-0.1% of Nb and the like, the carbon equivalent is controlled to be 3.6%-3.8%, the graphite form is A + B type, and a nanoscale VC + NbC composite dispersion strengthening phase and an epsilon-Cu precipitation phase are dispersed in a matrix; the preparation technology comprises the steps of raw material pretreatment and precise burdening, electric arc furnace smelting and vacuum refining, compound inoculation and A + B type graphite induction, lost foam casting and gradient cooling, double-stage heat treatment and the like, and optional defect repairing. According to the alloy cast iron and the preparation method thereof, through multi-element synergistic strengthening and precise process regulation and control, the tensile strength of the alloy cast iron is larger than or equal to 380 MPa, the impact toughness ak is larger than or equal to 22 J / cm < 2 >, the quenching hardness is 55-58 HRC, the alloy cast iron has high hardness and high toughness, the problem that a traditional material is strong and brittle is solved, the high-frequency stamping requirement of an automobile drawing die is met, the performance is stable, and the industrial adaptability is high.

Description

Technical Field

[0001] This invention belongs to the field of automotive mold material technology, and in particular relates to a high-hardness, high-toughness MoCr alloy cast iron for automotive drawing dies and its preparation process. Background Technology

[0002] Automotive drawing dies are core equipment in automobile body forming. During their service, they must withstand high-frequency stamping loads, complex stress distributions, and severe friction, placing stringent requirements on the strength, toughness, and wear resistance of the materials. Currently, the mainstream die materials in the industry are mainly divided into two categories: one is traditional MoCr alloy cast iron (such as GM241), whose matrix structure has uneven pearlite distribution, low carbide content, and coarse morphology. The graphite is a single coarse flake type A graphite, resulting in a tensile strength of only 310-330MPa and an impact toughness of less than 15J / cm². 2 Dies are prone to cracking and chipping when stamping complex-shaped workpieces, thus limiting their service life. Another type is high-grade alloy ductile iron, represented by GGG70L, which, although possessing a tensile strength exceeding 350 MPa, has an impact toughness of approximately 18 J / cm². 2 However, thick-walled castings are prone to spheroidization degradation, leading to performance fluctuations and making them difficult to adapt to long-term stable use under complex working conditions.

[0003] To address the industry's pain point of "synergistic optimization of high strength and high toughness," attempts have been made to improve the situation through optimizing alloy element ratios and adjusting heat treatment processes. However, existing solutions often focus on improving a single performance aspect, making it difficult to achieve simultaneous breakthroughs in both strength and toughness. Some processes increase strength by increasing carbide content, but this leads to a significant decrease in toughness; others improve toughness by adjusting cooling rates, but at the expense of the high strength and wear resistance required for the molds. Therefore, developing a composite alloy cast iron for automotive drawing dies with innovative composition design, strong process adaptability, and both high toughness and high strength, along with its preparation process, has become a pressing technical challenge for the industry. Summary of the Invention

[0004] To address the problems in the prior art, the present invention proposes the following technical solution: A MoCr alloy cast iron for high-hardness and high-toughness automotive drawing dies. The composition by mass percentage is as follows: C: 3.1-3.3%, Si: 1.6-1.8%, Mn: 0.7-0.9%, Cr: 0.8-1.0%, Mo: 0.3-0.4%, Cu: 0.2-0.3%, V: 0.1-0.2%, Nb: 0.05-0.1%, S≤0.08%, P≤0.07%, with the balance being Fe; The carbon equivalent CE = C + Si / 3 + Mn / 6 is 3.6-3.8%; the graphite morphology of the alloy cast iron is A+B type, the ratio of type A graphite to type B graphite is 7:3-6:4, the graphite grade is ≥6, and the graphite sheet thickness is ≤2μm; the matrix contains a nano-sized "VC+NbC" composite dispersion strengthening phase and an ε-Cu precipitate phase, wherein the VC particle size is 5-20nm, the NbC particle size is 3-15nm, and the ε-Cu precipitate phase size is 20-50nm.

[0005] As a preferred embodiment of the above technical solution, the mechanical properties of the alloy cast iron meet the following requirements: tensile strength ≥ 380 MPa, impact toughness ak ≥ 22 J / cm. 2 The Brinell hardness is ≥240HB, and the quenching hardness is 55-58HRC.

[0006] A manufacturing process for the aforementioned high-hardness, high-toughness MoCr alloy cast iron for automotive drawing dies includes the following steps: Step 1, Raw Material Pretreatment and Precise Batching: Select high-purity pig iron with a purity ≥99.7%, low-phosphorus scrap steel with P ≤0.06%, ferrochrome with a Cr content ≥68%, ferromolybdenum with a Mo content ≥55%, electrolytic copper, ferrovanadium with a V content ≥50%, and ferroniobium with a Nb content ≥60% as raw materials; crush the raw materials into 40-60mm blocks, remove surface oxide scale and oil stains by shot blasting, and then batch them according to the component ratio described in claim 1, with a batching error ≤±0.05%; Step 2, Electric Arc Furnace Melting and Vacuum Refining: The pretreated raw materials are put into the electric arc furnace and heated to 1350-1380℃ and held for 20-25 minutes. CaO-Al2O3 composite slagging agent is added (the amount added is 0.8-1.0% of the total amount of molten iron), and stirred for 10-15 minutes to remove P and S impurities. The vacuum refining system is turned on and the vacuum degree is controlled at 0.02-0.04MPa. The refining is carried out for 20-25 minutes to remove H and O gas impurities. The temperature is further increased to 1500-1530℃, and ferrochrome, ferromolybdenum, electrolytic copper, ferrovanadium, and ferroniobium are added. The mixture is stirred at a speed of 40 r / min for 15-20 minutes to obtain a composite alloy molten iron with uniform composition. Step 3, Composite Inoculation and A+B Type Graphite Induction: 10-15 minutes before the alloy molten iron is tapped from the furnace, add the composite inoculator and Ce-Mg composite modifier, and stir rapidly for 10-12 minutes; the composite inoculator consists of 50% SiC, 30% TiC, and 20% Zr, with a particle size of 1-2 mm, and the addition amount is 0.6-0.7% of the total molten iron; the Ce-Mg composite modifier is added at 0.2-0.3% of the total molten iron. Step 4, Lost Foam Casting and Gradient Cooling: A lost foam mold is used, with the cavity surface coated with a refractoriness ≥1800℃ and a coating thickness of 1.5-2.0mm. The pouring parameters are adjusted according to the casting wall thickness: for a wall thickness of 30-60mm, the pouring temperature is 1360-1390℃ and the pouring speed is 1.0-1.2m / s; for a wall thickness of 60-100mm, the pouring temperature is 1390-1420℃ and the pouring speed is 0.8-1.0m / s. A top-pour gating system is used, combined with multi-point venting channels on the side. After pouring, the casting is naturally kept at the desired temperature for 5-7 hours. When the surface temperature of the casting drops to 700-750℃, it is transferred to a gradient cooling box, successively kept at 500-550℃ for 12-15 hours, then at 300-350℃ for 8-10 hours, and finally naturally cooled to room temperature. Step 5, Two-stage heat treatment: Heat the casting to 890-930℃ at a heating rate of 4-6℃ / min and hold for 3-4 hours (extend the holding time by 0.6 hours for every 10mm increase in wall thickness); use an oil mist-air cooling composite cooling method, first cool to 300-350℃ at a rate of 80-100℃ / min, then switch to air cooling to 200-220℃; then heat to 220-240℃ and hold for 2.5-3 hours, then cool with the furnace to below 150℃; then heat to 180-200℃ and hold for 2-2.5 hours, then cool with the furnace to room temperature.

[0007] As a preferred embodiment of the above technical solution, in step 2, bottom-blown argon gas is used for stirring during the electric arc furnace melting process, with an argon gas flow rate of 1.0-1.5 L / min.

[0008] As a preferred embodiment of the above technical solution, in step 3, the composite inoculant and Ce-Mg composite modifier are added in batches: first, 70% of the inoculant and modifier are added, and after stirring for 5 minutes, the remaining 30% is added.

[0009] As a preferred embodiment of the above technical solution, in step 4, the diameter of the side venting channel of the lost foam mold is 8-10mm, and the channel spacing is 50-80mm.

[0010] As a preferred embodiment of the above technical solution, in step 5, the oil temperature for oil mist cooling is 35-45℃, the oil pressure is 0.5-0.7MPa, and the air cooling speed is 2-3m / s.

[0011] As a preferred embodiment of the above technical solution, the defect repair steps are also included: welding repair is performed using Ni-Cr-Mo alloy welding rods (diameter 3.2-4.0mm), with a welding current of 100-130A, a welding voltage of 21-23V, and a welding speed of 4-6cm / min; the casting is preheated to 200-250℃ before welding, and then slowly cooled at 380-420℃ for 3-4 hours after welding.

[0012] The beneficial effects of this invention are as follows: 1. An innovative "V+Nb+Cu+Cr" multi-element synergistic strengthening system is constructed. V and Nb form a nanoscale "VC+NbC" composite dispersed strengthening phase, significantly improving the matrix strength. Cu improves the matrix toughness simultaneously through solid solution strengthening and ε-Cu precipitation strengthening. Combined with Cr to promote uniform carbide distribution and the structural advantages of A+B type graphite, the material achieves a tensile strength ≥380MPa and an impact toughness ak ≥22J / cm. 2 With a Brinell hardness ≥240HB and a quenching hardness of 55-58HRC, its strength and toughness are improved by more than 30% compared with traditional MoCr alloy cast iron, and its toughness is improved by more than 20% compared with GGG70L high-grade ductile iron. It completely solves the industry pain point of traditional materials being "strong but brittle" and is suitable for high-frequency stamping requirements under complex working conditions. 2. A+B type graphite, formed through component ratio and process control, achieves a precise balance between toughness and strength. Type A graphite is uniformly distributed in flakes, which can effectively alleviate stress concentration during the stamping process and improve the material's impact fracture resistance. Type B graphite is fine and dense, with high bonding strength with the matrix, which significantly improves the material's density and wear resistance. The two work together to make the mold less prone to cracking when subjected to complex stresses, while also reducing surface wear and extending the service life. 3. Employing an electric arc furnace melting and vacuum refining process, the purity of the molten iron reaches an industry-leading level (gas content ≤0.003%), effectively avoiding the adverse effects of impurities on the formation of V and Nb carbides; the lost foam casting and gradient cooling processes are suitable for castings of different wall thicknesses (30-100mm), solving the problem of microstructure segregation in thick-walled castings; the dual-stage heat treatment process precisely controls the formation and distribution of the "VC+NbC" composite strengthening phase and the ε-Cu precipitate phase, keeping the performance fluctuation range of the casting within ±1.5%, and achieving a casting qualification rate of over 99%, enabling large-scale production without modifying existing production lines; moreover, the prepared composite alloy cast iron is not only suitable for automotive body panel drawing dies, but can also be extended to automotive structural parts, engineering machinery stamping dies, and other fields, especially suitable for the high-precision production needs of thick-walled and complex-shaped dies, providing the die manufacturing industry with a new material solution that combines high toughness and industrial adaptability. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments.

[0014] (a) Composition design of composite alloy cast iron (by mass percentage) C: 3.1-3.3%, Si: 1.6-1.8%, Mn: 0.7-0.9%, Cr: 0.8-1.0%, Mo: 0.3-0.4%, Cu: 0.2-0.3%, V: 0.1-0.2%, Nb: 0.05-0.1%, S≤0.08%, P≤0.07%, with the balance being Fe; wherein, the carbon equivalent CE = C + Si / 3 + Mn / 6 is controlled at 3.6-3.8%, through the elemental composition of Cr, V, Nb, and Cu. Multiple synergistic effects construct a performance enhancement system of "dispersion strengthening + matrix toughening": Cr promotes uniform precipitation of carbides, V and Nb form fine carbonitrides to achieve dispersion strengthening, Cu element improves matrix toughness through solid solution strengthening and microstructure optimization, and the four elements work together to achieve a precise balance between strength and toughness; in particular, through the precise ratio of Si and S elements, Si provides the basic conditions for graphite nucleation, and S element moderately blunts the graphite growth rate, laying the compositional foundation for the formation of A+B type graphite morphology.

[0015] 1. The working principle and effects of element V Mechanism of action: Vanadium (V) is a strong carbide-forming element with an extremely strong affinity for carbon (C). During smelting and heat treatment, V preferentially combines with C to form vanadium carbide (VC). VC has a wide formation temperature range (800-1200℃) and exhibits excellent stability at high temperatures (melting point as high as 2830℃). VC has a face-centered cubic crystal structure, forming a good coherent relationship with the iron matrix, with low lattice mismatch, effectively hindering dislocation movement. Simultaneously, the atomic radius of V (0.132 nm) is similar to that of Fe (0.127 nm), allowing some V atoms that do not form carbides to dissolve in the iron matrix, further enhancing the matrix strength through solid solution strengthening. Furthermore, V can refine austenite grains, inhibiting grain growth during heating and creating conditions for the subsequent formation of fine and uniform martensite.

[0016] Performance and effects: By controlling the amount of V added to 0.1-0.2%, nano-sized VC particles with a size of 5-20nm can be formed and uniformly dispersed in the matrix, which increases the tensile strength of the material by 25-30MPa; at the same time, the grain size is refined to below 50μm, reducing grain boundary defects and increasing the impact toughness of the material by 15-20%, effectively avoiding stress concentration cracking caused by coarse grains in the mold during high-frequency stamping.

[0017] 2. The working principle and effects of Nb element Mechanism of action: Similar to V, Nb is a strong carbide-forming element, but its bonding force with C is stronger, resulting in NbC (niobium carbide) with higher stability (melting point 3490℃). Furthermore, its solubility in the matrix is ​​extremely low, almost unaffected by temperature changes. NbC particles are smaller (3-15nm), exhibiting superior dispersion and hindering dislocation movement and grain growth over a wider temperature range. Simultaneously, Nb can combine with N in molten steel to form NbN, synergistically enhancing the dispersion strengthening effect. In addition, Nb significantly increases the nucleation rate of carbides, promoting uniform carbide distribution and avoiding the toughness reduction problem caused by carbide aggregation in traditional MoCr alloy cast iron.

[0018] Performance effects: When the amount of Nb added is controlled at 0.05-0.1%, it forms a "VC+NbC" composite dispersion strengthening phase with V. The synergistic effect of the two increases the yield strength of the material by 30-35 MPa and the tensile strength by 15-20 MPa. At the same time, NbC particles can pin grain boundaries and inhibit grain boundary sliding, which significantly improves the high-temperature stability of the material. It maintains the stability of the structure during mold heat treatment and service, and the performance fluctuation range is controlled within ±1.5%.

[0019] 3. The working principle and effects of Cu element Mechanism of action: Cu has a certain solid solubility in an iron matrix (approximately 0.2% at room temperature). When the addition amount is controlled at 0.2-0.3%, most Cu atoms are dissolved in the iron matrix, enhancing the matrix strength through solid solution strengthening. Simultaneously, the presence of Cu atoms alters the electronic structure of the iron matrix, reducing dislocation movement resistance and improving the matrix's plasticity and toughness. During heat treatment, some Cu atoms precipitate as fine ε-Cu precipitates, with a size of 20-50 nm. This precipitate does not significantly reduce the matrix's toughness and further enhances strength through precipitation strengthening. Furthermore, Cu can improve the bonding state between graphite and the matrix, reducing stress concentration at the graphite-matrix interface and improving the material's impact resistance.

[0020] Performance effect: The addition of Cu element increases the impact toughness of the material by 20-25%, reaching 22 J / cm. 2 In addition, it increases the elongation at break of the material by 5-8%, effectively alleviating the contradiction of "strong but brittle" in traditional high-strength alloy cast iron; the precipitation of ε-Cu phase increases the tensile strength of the material by an additional 10-15MPa, achieving simultaneous optimization of strength and toughness, ensuring that the mold has sufficient strength to resist deformation and good toughness to avoid cracking when subjected to complex stamping stress.

[0021] (II) Preparation process steps Raw material pretreatment and precise batching: High-purity pig iron (purity ≥99.7%), low-phosphorus scrap steel (P≤0.06%), ferrochrome (Cr content ≥68%), ferromolybdenum (Mo content ≥55%), electrolytic copper, ferrovanadium (V content ≥50%), and ferroniobium (Nb content ≥60%) are selected as raw materials. The raw materials are crushed into 40-60mm blocks, and the surface oxide scale and oil stains are removed by shot blasting. The raw materials are precisely batched according to the above component ratios, and the batching error is controlled within ±0.05% to ensure that the content of key elements such as V, Nb, and Cu accurately meets the standards.

[0022] Electric arc furnace smelting and vacuum refining: The pretreated raw materials are put into the electric arc furnace and heated to 1350-1380℃, held for 20-25 minutes to allow the raw materials to initially melt and form molten iron. CaO-Al2O3 composite slagging agent is added to the furnace (the amount added is 0.8-1.0% of the total amount of molten iron), and stirred for 10-15 minutes to remove impurities such as P and S in the molten iron. The vacuum refining system is turned on, and the vacuum degree in the furnace is controlled at 0.02-0.04MPa. The refining time is 20-25 minutes to remove gaseous impurities such as H and O in the molten iron and avoid the combination of gas with V and Nb to form harmful inclusions. The temperature is further increased to 1500-1530℃, and ferrochrome, ferromolybdenum, electrolytic copper, ferrovanadium, and ferroniobium are added. The mixture is stirred at a speed of 40 r / min for 15-20 minutes to ensure that alloying elements such as V, Nb, and Cu are uniformly dissolved to obtain a composite alloy molten iron with uniform composition.

[0023] Composite inoculation and A+B type graphite-induced treatment: 10-15 minutes before the alloy molten iron is tapped from the furnace, add a composite inoculator (composed of 50% SiC, 30% TiC, and 20% Zr, with a particle size of 1-2 mm) into the furnace, with an addition amount of 0.6-0.7% of the total molten iron; at the same time, add a Ce-Mg composite modifier (with an addition amount of 0.2-0.3% of the total molten iron) and stir rapidly for 10-12 minutes.

[0024] The formation principle of A+B type graphite: First, the decomposition of SiC and TiC in the composite inoculant produces a large number of fine carbon particles, providing sufficient nuclei for graphite nucleation. In the early stage of molten iron cooling (1400-1350℃), carbon atoms preferentially attach to the nucleation nuclei, forming uniformly distributed and regularly shaped A-type graphite. As the temperature of the molten iron continues to decrease (1350-1250℃), through the synergistic effect of the supercooling gradient (40-80℃) constructed by vacuum refining and the Ce-Mg composite modifier, Ce optimizes the graphite growth direction, while Mg reduces the graphite growth rate. Carbon atoms that did not participate in the formation of A-type graphite rapidly nucleate at grain boundaries and defects, forming fine and dense B-type graphite, ultimately forming an "A+B type composite graphite" structure. Among them, A-type graphite ensures the basic toughness of the material and avoids brittle fracture; B-type graphite refines the microstructure and improves the density and strength of the matrix. The two work together to achieve complementary properties.

[0025] Key to graphite morphology control: By controlling the carbon equivalent to 3.6-3.8%, graphite coarsening caused by excessive carbon equivalent is avoided. At the same time, Si (1.6-1.8%) promotes graphite nucleation, while S (≤0.08%) moderately inhibits excessive graphite growth. This ensures that the ratio of type A graphite to type B graphite is controlled at 7:3-6:4, the graphite grade is ≥6, and the graphite sheet thickness is ≤2μm, significantly improving the bonding strength between graphite and the matrix.

[0026] Lost foam casting and gradient cooling: Lost foam molds are used, and the mold cavity surface is coated with a high refractoriness coating (refractory ≥1800℃) with a coating thickness of 1.5-2.0mm; the pouring parameters are adjusted according to the casting wall thickness: when the wall thickness is 30-60mm, the pouring temperature is 1360-1390℃ and the pouring speed is 1.0-1.2m / s; when the wall thickness is 60-100mm, the pouring temperature is 1390-1420℃ and the pouring speed is 0.8-1.0m / s; a top-pouring gating system is used, combined with multiple venting channels on the side to ensure smooth gas discharge during the pouring process. After casting, the casting is naturally kept at the mold temperature for 5-7 hours. When the surface temperature of the casting drops to 700-750℃, the casting is removed and transferred to a gradient heat preservation box: In the first stage, the casting is cooled to 500-550℃ at a rate of 30-40℃ / h and held for 12-15 hours to promote the full formation of V and Nb carbides; in the second stage, the casting is cooled to 300-350℃ at a rate of 20-25℃ / h and held for 8-10 hours to reduce carbide segregation; in the third stage, the casting is naturally cooled to room temperature to completely eliminate casting internal stress, ensure that carbides, A+B type graphite and matrix are evenly distributed, and avoid graphite morphological distortion and elemental agglomeration.

[0027] Two-stage heat treatment: Quenching treatment: Place the casting cooled to room temperature into a box-type resistance furnace and heat it to 890-930℃ at a rate of 4-6℃ / min, and hold it for 3-4 hours (extend the holding time by 0.6 hours for every 10mm increase in wall thickness) to ensure that the V and Nb carbides are uniformly dispersed and distributed, and at the same time, to fully homogenize the austenite; adopt an oil mist-air cooling composite cooling method, first cool with oil mist at a rate of 80-100℃ / min to 300-350℃, and then switch to air cooling to 200-220℃ to avoid internal stress cracking caused by rapid cooling, ensure that the austenite is fully transformed into martensite, and at the same time protect the A+B type graphite structure and the "VC+NbC" composite strengthening phase from being damaged; Double tempering treatment: First tempering: The quenched casting is heated to 220-240℃ and held for 2.5-3 hours, then cooled to below 150℃ in the furnace to eliminate quenching internal stress and promote the precipitation of ε-Cu phase, achieving precipitation strengthening; Second tempering: The temperature is raised to 180-200℃ and held for 2-2.5 hours, then cooled to room temperature in the furnace to further stabilize the martensitic structure and the "VC+NbC" composite strengthening phase, eliminate residual stress, improve the toughness of the material, and strengthen the bonding force between A+B type graphite and the matrix.

[0028] Defect Repair (Optional): If the casting has local defects, it can be repaired by welding with Ni-Cr-Mo alloy welding rods (diameter 3.2-4.0mm). The welding current is 100-130A, the voltage is 21-23V, and the welding speed is 4-6cm / min. Before welding, the casting should be preheated to 200-250℃. After welding, it should be kept at 380-420℃ for 3-4 hours for slow cooling to ensure good fusion between the welded area and the matrix structure. The graphite morphology in the welded area should still maintain the A+B type structure, and the V, Nb, and Cu elements should be evenly distributed and the performance consistency should meet the standards.

[0029] II. Optimal Solution In step 2, bottom-blown argon gas is used for stirring during electric arc furnace melting, with an argon gas flow rate of 1.0-1.5 L / min. This further improves the uniformity of alloy element distribution and the purity of molten iron, creating conditions for the uniform nucleation of V and Nb carbides and the uniform formation of A+B type graphite.

[0030] In step 3, the composite inoculant and Ce-Mg composite modifier are added in batches. First, 70% of the inoculant and modifier are added and stirred for 5 minutes before the remaining 30% is added to ensure that they are fully dissolved in the molten iron. This precisely controls the formation ratio of A+B type graphite and avoids adverse effects on the formation of V and Nb carbides.

[0031] In step 5, the oil temperature for oil mist cooling is controlled at 35-45℃, the oil pressure at 0.5-0.7MPa, and the air cooling speed at 2-3m / s to ensure a uniform and stable cooling process, avoid morphological changes in A+B type graphite due to uneven cooling, and at the same time ensure the stability of the morphology and distribution of the "VC+NbC" composite reinforcing phase and the ε-Cu precipitate phase.

[0032] Example 1: Preparation of a 40mm thick automotive front fender drawing die casting Raw material ratio (mass percentage): C 3.2%, Si 1.7%, Mn 0.8%, Cr 0.9%, Mo 0.35%, Cu 0.25%, V 0.15%, Nb 0.08%, S 0.06%, P 0.05%, balance Fe; carbon equivalent CE = 3.2 + 1.7 / 3 + 0.8 / 6 ≈ 3.7%.

[0033] Preparation process: Raw material pretreatment: Select pig iron with a purity of 99.8%, scrap steel containing 0.05% P and corresponding alloy raw materials, crush them into 50mm blocks, and shot blast to remove impurities; Electric arc furnace smelting: Put the product into the electric arc furnace, heat it to 1360℃ and hold it for 22 minutes, add CaO-Al2O3 composite slagging agent (0.9%), and stir for 12 minutes; vacuum refining (0.03MPa, 22 minutes), heat it to 1510℃ and add alloying elements, and stir for 18 minutes; Inoculation and A+B type graphite induction: 12 minutes before tapping, add composite inoculant (0.65%) and Ce-Mg composite modifier (0.25%) in batches and stir for 11 minutes to adjust the ratio of type A graphite to type B graphite to 6.5:3.5. Casting: Lost foam casting mold, coating thickness 1.8mm, casting temperature 1370℃, casting speed 1.1m / s; Gradient cooling: Insulate the mold for 6 hours, then cool in a gradient insulation box (720℃→520℃ for 13 hours→320℃ for 9 hours→room temperature); Heat treatment: Quenching at 910℃ for 3.4h, followed by oil mist-air cooling to 210℃; first tempering at 230℃ for 2.8h, and second tempering at 190℃ for 2.2h.

[0034] Performance testing: Organization: The graphite morphology is type A+B (type A accounts for 65%, type B accounts for 35%), graphite grade is 6.5, and the thickness of the graphite flakes is 1.5μm; the carbides are uniformly distributed with a content of 14.2%, the pearlite accounts for 90%, the "VC+NbC" composite reinforcing phase is uniformly dispersed (VC particle size 8-12nm, NbC particle size 5-10nm), and the ε-Cu precipitate phase size is 30-40nm; Properties: Tensile strength 392 MPa, impact toughness ak = 23.5 J / cm 2 Brinell hardness 246HB, quenching hardness 56HRC; Service testing: After 50,000 stamping tests, the mold showed no cracking or chipping, and the surface wear was ≤0.02mm, meeting the requirements for use of automotive outer panel drawing dies.

[0035] Example 2: Preparation of a 90mm thick automobile roof drawing die casting Raw material ratio (mass percentage): C 3.3%, Si 1.8%, Mn 0.9%, Cr 1.0%, Mo 0.4%, Cu 0.3%, V 0.2%, Nb 0.1%, S 0.07%, P 0.06%, balance Fe; carbon equivalent CE = 3.3 + 1.8 / 3 + 0.9 / 6 ≈ 3.8%.

[0036] Preparation process: Raw material pretreatment: The raw material is crushed into 60mm blocks and shot blasted to ensure that the surface is free of impurities; Electric arc furnace smelting: heat to 1380℃ and hold for 25 min, add CaO-Al2O3 composite slagging agent (1.0%), stir for 15 min; vacuum refining (0.04MPa, 25 min), heat to 1530℃ and add alloying elements, stir for 20 min; Inoculation and A+B type graphite induction: 15 minutes before tapping, add composite inoculant (0.7%) and Ce-Mg composite modifier (0.3%) in batches and stir for 12 minutes to adjust the ratio of type A graphite to type B graphite to 6:4. Casting: Lost foam casting mold, coating thickness 2.0mm, casting temperature 1410℃, casting speed 0.9m / s; Gradient cooling: Insulate the mold for 7 hours, then cool in a gradient insulation box (750℃→550℃ for 15 hours→350℃ for 10 hours→room temperature); Heat treatment: Quenching at 930℃ for 5.4h (90mm wall thickness: 3h + (90-30) / 10×0.6h = 5.4h), oil mist-air cooling composite cooling to 220℃; first tempering at 240℃ for 3h, second tempering at 200℃ for 2.5h.

[0037] Performance testing: Microstructure: The graphite morphology is type A+B (type A accounts for 60%, type B accounts for 40%), graphite grade 6, graphite sheet thickness 1.8μm; carbide content 15.1%, uniformly distributed without grain boundary aggregation, "VC+NbC" composite strengthening phase dispersedly distributed (VC particle size 10-15nm, NbC particle size 8-12nm), ε-Cu precipitate phase size 40-50nm; Properties: Tensile strength 386 MPa, impact toughness ak = 22.3 J / cm 2 Brinell hardness 252HB, quenching hardness 57HRC; Internal quality: Ultrasonic testing showed no defects ≥ φ1mm, good microstructure uniformity, and met the strength and toughness requirements for thick-walled drawing dies.

[0038] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it.

Claims

1. A MoCr alloy cast iron for high-hardness and high-toughness automotive drawing dies, characterized in that: The composition by mass percentage is as follows: C: 3.1-3.3%, Si: 1.6-1.8%, Mn: 0.7-0.9%, Cr: 0.8-1.0%, Mo: 0.3-0.4%, Cu: 0.2-0.3%, V: 0.1-0.2%, Nb: 0.05-0.1%, S≤0.08%, P≤0.07%, with the balance being Fe; The carbon equivalent CE = C + Si / 3 + Mn / 6 is 3.6-3.8%; the graphite morphology of the alloy cast iron is A+B type, the ratio of type A graphite to type B graphite is 7:3-6:4, the graphite grade is ≥6, and the graphite sheet thickness is ≤2μm; the matrix contains a nano-sized "VC+NbC" composite dispersion reinforcing phase and an ε-Cu precipitate phase, wherein the VC particle size is 5-20nm, the NbC particle size is 3-15nm, and the ε-Cu precipitate phase size is 20-50nm.

2. The MoCr alloy cast iron according to claim 1, characterized in that, The mechanical properties of the alloy cast iron meet the following requirements: tensile strength ≥ 380 MPa, impact toughness ak ≥ 22 J / cm. 2 The Brinell hardness is ≥240HB, and the quenching hardness is 55-58HRC.

3. A preparation process for the high-hardness, high-toughness MoCr alloy cast iron for automotive drawing dies as described in claim 1, characterized in that, Includes the following steps: Step 1, Raw Material Pretreatment and Precise Batching: Select high-purity pig iron with a purity ≥99.7%, low-phosphorus scrap steel with P ≤0.06%, ferrochrome with a Cr content ≥68%, ferromolybdenum with a Mo content ≥55%, electrolytic copper, ferrovanadium with a V content ≥50%, and ferroniobium with a Nb content ≥60% as raw materials; crush the raw materials into 40-60mm blocks, remove surface oxide scale and oil stains by shot blasting, and then batch them according to the component ratio described in claim 1, with a batching error ≤±0.05%; Step 2, Electric Arc Furnace Melting and Vacuum Refining: The pretreated raw materials are put into the electric arc furnace and heated to 1350-1380℃ and held for 20-25 minutes. CaO-Al2O3 composite slagging agent is added (the amount added is 0.8-1.0% of the total amount of molten iron), and stirred for 10-15 minutes to remove P and S impurities. The vacuum refining system is turned on and the vacuum degree is controlled at 0.02-0.04MPa. The refining is carried out for 20-25 minutes to remove H and O gas impurities. The temperature is further increased to 1500-1530℃, and ferrochrome, ferromolybdenum, electrolytic copper, ferrovanadium, and ferroniobium are added. The mixture is stirred at a speed of 40 r / min for 15-20 minutes to obtain a composite alloy molten iron with uniform composition. Step 3, Composite Inoculation and A+B Type Graphite Induction: 10-15 minutes before the alloy molten iron is tapped from the furnace, add the composite inoculator and Ce-Mg composite modifier, and stir rapidly for 10-12 minutes; the composite inoculator consists of 50% SiC, 30% TiC, and 20% Zr, with a particle size of 1-2 mm, and the addition amount is 0.6-0.7% of the total molten iron; the Ce-Mg composite modifier is added at 0.2-0.3% of the total molten iron. Step 4, Lost Foam Casting and Gradient Cooling: A lost foam mold is used, with the cavity surface coated with a refractoriness ≥1800℃ and a coating thickness of 1.5-2.0mm. The pouring parameters are adjusted according to the casting wall thickness: for a wall thickness of 30-60mm, the pouring temperature is 1360-1390℃ and the pouring speed is 1.0-1.2m / s; for a wall thickness of 60-100mm, the pouring temperature is 1390-1420℃ and the pouring speed is 0.8-1.0m / s. A top-pour gating system is used, combined with multi-point venting channels on the side. After pouring, the casting is naturally kept at the desired temperature for 5-7 hours. When the surface temperature of the casting drops to 700-750℃, it is transferred to a gradient cooling box, successively kept at 500-550℃ for 12-15 hours, then at 300-350℃ for 8-10 hours, and finally naturally cooled to room temperature. Step 5, Two-stage heat treatment: Heat the casting to 890-930℃ at a heating rate of 4-6℃ / min and hold for 3-4 hours (extend the holding time by 0.6 hours for every 10mm increase in wall thickness); use an oil mist-air cooling composite cooling method, first cool to 300-350℃ at a rate of 80-100℃ / min, then switch to air cooling to 200-220℃; then heat to 220-240℃ and hold for 2.5-3 hours, then cool with the furnace to below 150℃; then heat to 180-200℃ and hold for 2-2.5 hours, then cool with the furnace to room temperature.

4. The preparation process according to claim 3, characterized in that, In step 2, bottom-blown argon gas is used for stirring during the electric arc furnace melting process, with an argon gas flow rate of 1.0-1.5 L / min.

5. The preparation process according to claim 3, characterized in that, In step 3, the compound inoculant and Ce-Mg compound modifier are added in batches: first add 70% of the inoculant and modifier, stir for 5 minutes, and then add the remaining 30%.

6. The preparation process according to claim 3, characterized in that, In step 4, the diameter of the side venting channel of the lost foam mold is 8-10mm, and the channel spacing is 50-80mm.

7. The preparation process according to claim 3, characterized in that, In step 5, the oil temperature for oil mist cooling is 35-45℃, the oil pressure is 0.5-0.7MPa, and the air cooling speed is 2-3m / s.

8. The preparation process according to claim 3, characterized in that, It also includes defect repair steps: use Ni-Cr-Mo alloy welding rods (diameter 3.2-4.0mm) for welding repair, welding current 100-130A, welding voltage 21-23V, welding speed 4-6cm / min; preheat the casting to 200-250℃ before welding, and keep it at 380-420℃ for 3-4 hours for slow cooling after welding.