High thermal conductive die steel for aluminum alloy die casting and preparation process thereof

Abstract

The application discloses a high-thermal-conductivity die steel for aluminum alloy die casting and a preparation process thereof, and relates to the technical field of die steel. The chemical components of the die steel are as follows in terms of percentage by mass: C 0.20-0.26%, Si 0.05-0.12%, Mn 0.20-0.35%, Cr 0.80-1.20%, Mo 2.0-2.5%, Ni 2.8-3.5%, Al 0.30-0.50%, Cu 0.60-1.20%, V 0.05-0.12%, Co 0.50-1.00%, Nb 0.01-0.03%, RE 0.01-0.05%, and the balance of Fe, wherein RE is mixed rare earth with a mass ratio of Ce to La being 2:1. The die steel prepared by the application has a room-temperature thermal conductivity of 33-36 W / (m*K), a 500 DEG C thermal conductivity of 42-47 W / (m*K), and a hardness of 48-49 HRC.

Description

Technical Field

[0001] This invention relates to the field of mold steel technology, specifically to a high thermal conductivity mold steel for aluminum alloy die casting and its preparation process. Background Technology

[0002] During service, aluminum alloy die-casting molds are subjected to repeated high-temperature thermal shocks of 500–650°C on the cavity surface. The thermal conductivity of the mold steel directly affects the mold's heat dissipation efficiency and service life. Insufficient thermal conductivity prevents heat from being dissipated in time, keeping the mold cavity surface at a high temperature for extended periods. This accelerates the initiation and propagation of thermal fatigue cracks, leading to premature mold failure.

[0003] H13 steel is the most widely used aluminum alloy die-casting mold steel. Its room temperature thermal conductivity is approximately 25 W / (m·K), and its thermal conductivity at 500℃ is approximately 28.5–30.3 W / (m·K), which is insufficient to meet the requirements of high-efficiency die-casting production. The lattice distortion of the iron matrix caused by alloying elements is the main reason for the reduced thermal conductivity, with Si (Si) causing particularly severe damage.

[0004] Chinese patent CN121451067A discloses a steel for aluminum alloy die casting molds with high thermal conductivity and resistance to aluminum melt corrosion. Its composition is 1.4%–1.8% C, 0.8%–1.2% Si, 0.5%–1.0% Mn, and 1.5%–2.5% Mo. The microstructure contains fine graphite spheres, utilizing the high thermal conductivity of graphite to improve thermal conductivity. However, its excessively high carbon content severely impairs toughness and resistance to thermal fatigue, and the high Si content (0.8%–1.2%) adversely affects thermal conductivity. Furthermore, the graphitization process is difficult to control.

[0005] Chinese patent CN119082605A discloses a high thermal conductivity hot stamping die steel, employing a composition design approach of low Si (0.03%–0.15%), low Cr (0.10%–0.30%), high Mo (3.00%–3.30%), and high Ni (2.00%–2.30%). This approach reduces lattice distortion by lowering Si and Cr, but high thermal conductivity requires low solid solution of alloying elements, while mechanical properties depend on sufficient solid solution strengthening and precipitation strengthening, creating a fundamental contradiction.

[0006] In addition, in the field of mold steel heat treatment, retained austenite is generally regarded as an unfavorable structure and is eliminated as much as possible through tempering or deep cryogenic treatment to avoid martensitic transformation during use, which would lead to dimensional changes.

[0007] In summary, existing technologies have not yet resolved the contradiction between high thermal conductivity and good mechanical properties. Summary of the Invention

[0008] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present invention provide a high thermal conductivity mold steel for aluminum alloy die casting and its preparation process. The technical problem to be solved is to overcome the defect of insufficient thermal conductivity of existing aluminum alloy die casting mold steel at service temperature of 500-650°C.

[0009] To achieve the above objectives, the present invention provides the following technical solution: (a) Ingredient Design The chemical composition of the mold steel of this invention, by mass percentage, is as follows: The composition is as follows: C 0.20–0.26%, Si 0.05–0.12%, Mn 0.20–0.35%, Cr 0.80–1.20%, Mo 2.0–2.5%, Ni 2.8–3.5%, Al 0.30–0.50%, Cu 0.60–1.20%, V 0.05–0.12%, Co 0.50–1.00%, Nb 0.01–0.03%, RE 0.01–0.05%, with the balance being Fe and unavoidable impurities, wherein the RE is a mixture of rare earth elements with a Ce to La mass ratio of 2:1; P ≤ 0.015%, S ≤ 0.005%.

[0010] The functions and content ranges of each element are as follows: C 0.20~0.26%: Carbon is a basic element that ensures the strength of martensite. Low-carbon design can reduce the total amount of carbides and reduce the negative impact on the thermal conductivity of the matrix.

[0011] Si 0.05~0.12%: Si has the most serious impact on thermal conductivity. This invention significantly reduces the Si content to below 0.12%, reducing the obstruction to heat conduction from the source.

[0012] Mn 0.20~0.35%: Mn can ensure hardenability, but too much will impair thermal conductivity. It should be controlled within a moderate range.

[0013] Cr 0.80~1.20%: significantly lower than H13 steel, reducing the scattering of thermally conductive electrons by lattice distortion, while retaining basic hardenability and oxidation resistance.

[0014] Mo 2.0~2.5%: The main strengthening element, which forms a nanoscale Mo2C phase during tempering, providing a secondary hardening effect.

[0015] Ni 2.8–3.5%: Higher than the Ni content in conventional mold steels. Ni has a smaller negative impact on thermal conductivity than Cr and Si, and can effectively improve hardenability and toughness. The higher Ni content allows sufficient retained austenite to be retained after quenching. This retained austenite transforms into martensite in a stepwise and controllable manner during subsequent heat treatment and service. The thermal conductivity of newly formed martensite is higher than that of retained austenite (martensite thermal conductivity is about 35 W / (m·K), austenite thermal conductivity is about 21 W / (m·K)), and the phase transformation process absorbs heat, producing an auxiliary heat dissipation effect.

[0016] Al 0.30~0.50%: When the Al content is 0.37wt.%, the thermal conductivity of the Fe-Al alloy reaches a peak at a test temperature of 450℃, forming a synergistic effect with Cu and widening the high thermal conductivity temperature range.

[0017] Cu 0.60~1.20%: During tempering, Cu precipitates the ε-Cu phase, which scatters thermally conductive electrons much less than carbides of the same volume fraction. When the Cu content is 0.50wt.% and 1.03wt.%, the thermal conductivity of Cu-containing mold steel exhibits two peaks at a test temperature of 450℃. Furthermore, the precipitation temperature range of Cu differs from that of the Mo2C phase; this invention utilizes this difference to achieve time-sequential precipitation control.

[0018] V 0.05~0.12%: Added in trace amounts, only as an aid to grain refinement, to avoid excessive V carbides scattering thermal conductivity.

[0019] Co 0.50~1.00%: Co has a much smaller negative impact on thermal conductivity than Cr and Si, and can improve high-temperature strength.

[0020] Nb 0.01~0.03%: a strong carbide-forming element, forming nanoscale NbC, pinning grain boundaries and refining grains.

[0021] RE (Ce:La=2:1) ​​0.01~0.05%: Rare earth elements can deoxidize and desulfurize, spheroidize inclusions, and refine the as-cast structure.

[0022] (II) Preparation process The preparation process of the mold steel of this invention includes the following steps: S1 Rare Earth Pre-modification Treatment: At a steel refining temperature of 1550-1580℃, a mixture of rare earth elements with a Ce to La mass ratio of 2:1 is added at a ratio of 0.02-0.05% of the steel mass. The mixture is then electromagnetically stirred for 10-15 minutes, allowed to stand for 5-8 minutes to allow inclusions to float and be removed, and then electroslag remelted.

[0023] S2 High-Temperature Homogenization Treatment: The steel ingot after electroslag remelting is heated to 1220-1250℃ at a rate not exceeding 80℃ / h, held at that temperature for 20-25 hours, cooled to 850℃ at a cooling rate not exceeding 50℃ / h, and then air-cooled to room temperature.

[0024] S3 Multi-directional Forging: The steel ingot is forged alternately in three directions at 1150-1180℃, with a forging ratio of not less than 2 in each direction, a total forging ratio of not less than 6, and a final forging temperature of not less than 900℃. After forging, it is cooled to 650℃ at a cooling rate of not less than 50℃ / min, and then transferred to an annealing furnace for further cooling.

[0025] S4 Pre-heat treatment: Heat to 350℃ at a rate of 40℃ / h and hold for 4 hours, then heat to 620℃ at a rate of 50℃ / h and hold for 6 hours, then heat to 780℃ at a rate of 60℃ / h and hold for 3 hours, then heat to 880℃ at a rate of 100℃ / h and hold for 2 hours, and then air cool to room temperature.

[0026] The 350℃ holding stage induces Ni pre-segregation at grain boundaries, preparing for the quantitative locking of residual austenite during subsequent quenching. The 620℃ holding stage induces the pre-formation of Cu-rich clusters, which become preferential nucleation sites for subsequent ε-Cu phase precipitation. The 780℃ holding stage induces NbC pre-precipitation, pinning grain boundaries. The 880℃ holding stage induces the formation of AlN nanoparticles, further strengthening grain boundary pinning.

[0027] Of the four temperatures mentioned above, 350℃, 620℃, and 780℃ are all lower than the Ac1 temperature of this composition system (the Ac1 temperature of this composition system is 800–830℃). During the 880℃ holding stage, the temperature has entered above Ac1, and the matrix undergoes a partial austenite transformation (the austenite transformation amount is approximately 10–30%). During the subsequent air cooling to room temperature, this partial austenite transforms into a mixed structure of pearlite and bainite.

[0028] S5 Gradient solution treatment: Hold the forging at 850-880℃ for 1 hour, then at 940-970℃ for 1.5 hours, then cool it down to 900℃ at a cooling rate of 15℃ / min, then hold it at 1020-1040℃ for 1 hour, and finally hold it at 980-1000℃ for 0.5 hours.

[0029] In the 850–880℃ stage, VC-type carbides partially dissolve, while Cr-rich carbides remain largely insoluble. In the 940–970℃ stage, Cr-rich carbides begin to dissolve, while the Mo₂C phase is selectively retained. At 900℃, the grain boundary pinning effect of Al and the stable presence of NbC lock in the obtained austenite grain size. In the 1020–1040℃ stage, Cr, Mo, and V achieve complete solid solution. In the 980–1000℃ stage, some of the previously dissolved Mo and Cr reform nanoscale pre-precipitated clusters, becoming preferential nucleation sites for the subsequent Mo₂C phase.

[0030] S6 Isothermal Stage Quenching: The solution-treated forgings are isothermally held at 380–420℃ for 20–30 minutes, then isothermally held at 280–320℃ for 60–90 minutes, and finally isothermally held at 150–180℃ for 30 minutes. The Ms point of this composition system is approximately 320–350℃. After isothermal stage quenching, the forgings are transferred to cryogenic equipment within 30 minutes.

[0031] The 380–420℃ stage transforms some austenite into lower bainite (10–15%). The 280–320℃ stage transforms some austenite into martensite (50–60%), stopping the transformation and retaining untransformed austenite. The 150–180℃ stage transforms some of the retained austenite into martensite (5–8%), with the final retained austenite content controlled at 15–22%.

[0032] S7 Cryogenic Treatment: The quenched forgings are placed in an environment of -80 to -100℃ and held for 2 to 4 hours, then restored to room temperature at a heating rate of 20℃ / h. After cryogenic screening, the retained austenite content is reduced from 15 to 22% to 10 to 15%.

[0033] S8 Cyclic Phase Change Tempering: Heat to 470-500℃ at a heating rate of 150℃ / h and hold for 2.5 hours, then cool to 200℃ at a cooling rate of 60℃ / h; then heat to 520-550℃ at a heating rate of 120℃ / h and hold for 2 hours, then cool to 200℃ at a cooling rate of 50℃ / h; finally heat to 490-520℃ at a heating rate of 100℃ / h and hold for 1.5 hours, then cool to room temperature at a cooling rate of 30℃ / h.

[0034] The 470–500℃ stage preferentially precipitates the ε-Cu phase, which seizes nucleation sites. The 520–550℃ stage forces the Mo2C phase to precipitate at the remaining sites, resulting in a forced refinement of its size. The 490–520℃ stage finally transforms the untransformed retained austenite from the first two stages, while further homogenizing the size distribution of the ε-Cu and Mo2C phases.

[0035] S9 stabilization treatment: Cool to 180°C at a cooling rate of 8°C / min and hold for 6 hours, then cool to room temperature at a cooling rate of 15°C / min.

[0036] S10 Temperature Control Response Activation Process: Hold at 560-590℃ for 2 hours, then cool to 450℃ at a cooling rate of 200℃ / h, and then cool to room temperature at a cooling rate of 50℃ / h.

[0037] The present invention has the following beneficial effects: (1) By reducing the content of Si and Cr and optimizing the ratio of elements such as Ni, Al and Cu, this invention reduces the distortion effect of alloying elements on the iron matrix lattice and reduces the scattering of thermally conductive electrons, so that the room temperature thermal conductivity reaches 33-36 W / (m·K), which is comparable to the room temperature thermal conductivity of existing DHA-Thermo steel.

[0038] (2) This invention achieves a thermal conductivity of 42-47 W / (m·K) at 500℃ by continuously transforming the retained austenite into martensite at a service temperature of 500-650℃, taking advantage of the higher thermal conductivity of the newly formed martensite than that of the retained austenite and the endothermic nature of the phase transformation process. This is 25-30% higher than that of room temperature and 40-55% higher than that of H13 steel at 500℃.

[0039] (3) The retained austenite in this invention continuously and controllably transforms into martensite at the service temperature, forming a self-limiting temperature mechanism of "temperature rise → phase transformation heat absorption → enhanced thermal conductivity → heat dissipation → temperature drop", thereby maintaining the mold temperature within a relatively stable range and extending the mold service life.

[0040] (4) This invention comprehensively compensates for the strength that may be lost due to the reduction of Si and Cr by solid solution strengthening of Mo and precipitation strengthening of nano-sized Mo2C phase, precipitation strengthening of Cu ε-Cu phase, solid solution strengthening of Co and toughening effect of Ni, so that the hardness reaches 48-49HRC and the yield strength at 600℃ reaches 810-840MPa, which meets the requirements for use of aluminum alloy die casting molds.

[0041] (5) The present invention enables the rapid removal of heat from the surface of the mold cavity through high thermal conductivity, thereby reducing the peak temperature and temperature gradient in the thermal cycle and thus reducing the thermal stress amplitude; at the same time, the refined and dispersed nanoscale precipitates effectively hinder dislocation movement, delay the initiation and propagation of thermal fatigue cracks, and enable the thermal fatigue life to reach 2780 to 2920 cycles, which is 32 to 39% higher than that of H13 steel.

[0042] (6) This invention uses all mature industrial equipment, requires no special equipment, and has the conditions for direct industrialization. Attached Figure Description

[0043] Figure 1 This is a schematic flowchart of the mold steel preparation process of the present invention; Figure 2 This is a schematic diagram of the thermal conductivity-temperature curve of the mold steel of the present invention in the range of room temperature to 650°C; Figure 3 This is a schematic diagram of the precipitate distribution after the mold steel of the present invention undergoes S8 cycle phase transformation tempering. Detailed Implementation

[0044] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] Example 1

[0046] As attached Figures 1-3 As shown, the chemical composition of the mold steel in this embodiment, by mass percentage, is: C 0.23%, Si 0.08%, Mn 0.28%, Cr 1.0%, Mo 2.2%, Ni 3.0%, Al 0.37%, Cu 0.80%, V 0.08%, Co 0.70%, Nb 0.02%, RE (Ce:La=2:1) ​​0.03%, with the balance being Fe and unavoidable impurities; wherein the P content is 0.008% and the S content is 0.003%.

[0047] The preparation process is as follows: S1 Rare Earth Pre-modification Treatment: At a refining temperature of 1560℃, a mixture of rare earth elements with a Ce to La mass ratio of 2:1 is added at a ratio of 0.03% by mass of the molten steel. The mixture is then electromagnetically stirred for 12 minutes, allowed to stand for 6 minutes, and then electroslag remelted.

[0048] S2 High-Temperature Homogenization Treatment: The electroslag remelted steel ingot is heated to 1230℃ at a rate of 60℃ / h, held at that temperature for 22 hours, cooled to 850℃ at a rate of 40℃ / h, and then air-cooled to room temperature.

[0049] S3 Multi-directional Forging: The steel ingot is forged alternately in three directions at 1160℃, with a forging ratio of 2.5 per direction and a total forging ratio of 7.5. The final forging temperature is 920℃. After forging, the ingot is cooled to 650℃ at a cooling rate of 60℃ / min, and then transferred to an annealing furnace for further cooling.

[0050] S4 Pre-heat treatment: Heat to 350℃ at a rate of 40℃ / h and hold for 4 hours, then heat to 620℃ at a rate of 50℃ / h and hold for 6 hours, then heat to 780℃ at a rate of 60℃ / h and hold for 3 hours, then heat to 880℃ at a rate of 100℃ / h and hold for 2 hours, and then air cool to room temperature.

[0051] S5 gradient solution treatment: hold at 860℃ for 1 hour, then at 950℃ for 1.5 hours, then cool down to 900℃ at a cooling rate of 15℃ / min, then hold at 1030℃ for 1 hour, and finally hold at 990℃ for 0.5 hours.

[0052] S6 isothermal graded quenching: hold isothermally at 400℃ for 25 minutes, then at 300℃ for 75 minutes, and finally at 165℃ for 30 minutes. After isothermal graded quenching, transfer the forging to cryogenic equipment within 30 minutes.

[0053] S7 Cryogenic Treatment: Place in an environment of -90℃ for 3 hours, then restore to room temperature at a heating rate of 20℃ / h.

[0054] S8 Cyclic Phase Change Tempering: The first stage involves heating to 480℃ at a rate of 150℃ / h and holding for 2.5 hours, followed by cooling to 200℃ at a rate of 60℃ / h; the second stage involves heating to 530℃ at a rate of 120℃ / h and holding for 2 hours, followed by cooling to 200℃ at a rate of 50℃ / h; the third stage involves heating to 500℃ at a rate of 100℃ / h and holding for 1.5 hours, followed by cooling to room temperature at a rate of 30℃ / h.

[0055] S9 stabilization treatment: Cool to 180°C at a cooling rate of 8°C / min and hold for 6 hours, then cool to room temperature at a cooling rate of 15°C / min.

[0056] S10 Temperature Control Response Activation Process: Hold at 570℃ for 2 hours, then cool to 450℃ at a cooling rate of 200℃ / h, and then cool to room temperature at a cooling rate of 50℃ / h.

[0057] Example 2

[0058] The difference between this embodiment and Embodiment 1 lies in the adjustment of the components and some process parameters.

[0059] The chemical composition of the mold steel, by mass percentage, is as follows: C 0.25%, Si 0.10%, Mn 0.32%, Cr 1.1%, Mo 2.4%, Ni 3.2%, Al 0.42%, Cu 1.0%, V 0.10%, Co 0.85%, Nb 0.025%, RE (Ce:La=2:1) ​​0.04%, with the balance being Fe and unavoidable impurities; among which, the P content is 0.009% and the S content is 0.004%.

[0060] In step S2, the homogenization temperature is 1240℃, and the holding time is 24 hours. In step S5, the high-temperature solution treatment temperature is 1040℃. In step S6, the first isothermal temperature is 410℃, and the second isothermal temperature is 310℃. In step S8, the first step temperature is 490℃, and the second step temperature is 540℃. In step S10, the activation treatment temperature is 580℃. The remaining steps are the same as in Example 1.

[0061] Example 3

[0062] The difference between this embodiment and Embodiment 1 lies in the adjustment of the components and some process parameters.

[0063] The chemical composition of the mold steel, by mass percentage, is as follows: C 0.21%, Si 0.07%, Mn 0.25%, Cr 0.9%, Mo 2.1%, Ni 2.9%, Al 0.33%, Cu 0.70%, V 0.06%, Co 0.60%, Nb 0.015%, RE (Ce:La=2:1) ​​0.02%, with the balance being Fe and unavoidable impurities; the P content is 0.007%, and the S content is 0.003%.

[0064] In step S2, the homogenization temperature is 1225℃, and the holding time is 21 hours. In step S5, the high-temperature solution treatment temperature is 1025℃. In step S6, the first isothermal temperature is 390℃, and the second isothermal temperature is 290℃. In step S8, the first step temperature is 475℃, and the second step temperature is 525℃. In step S10, the activation treatment temperature is 565℃. The remaining steps are the same as in Example 1.

[0065] Comparative Example 1 This comparative example uses H13 steel, whose chemical composition by mass percentage is: C 0.38%, Si 1.0%, Mn 0.40%, Cr 5.2%, Mo 1.4%, V 1.0%, with the balance being Fe and unavoidable impurities. Conventional heat treatment was employed: austenitization at 1030℃ for 30 minutes, followed by oil quenching, and tempering twice at 580℃ for 2 hours each time.

[0066] Comparative Example 2 This comparative example uses the same chemical composition as Example 1 of the present invention, but adopts a conventional heat treatment process: directly heated to 1030°C for 1 hour to solidify, then oil quenched, and then tempered twice in a single temperature zone at 530°C for 2 hours each time.

[0067] Comparative Example 3 This comparative example uses DHA-Thermo steel, whose typical chemical composition by mass percentage is: C 0.38%, Si 0.5%, Mn 0.5%, Cr 3.0%, Mo 2.0%, V 0.8%, Ni 1.0%. The heat treatment process is: austenitization at 1030℃ followed by oil quenching, and tempering twice at 530-580℃, 2 hours each time.

[0068] Comparative Example 4 The difference between this comparative example and Example 1 is that in the S8 cyclic phase transformation tempering, the first step temperature is adjusted to 520–550°C and the second step temperature is adjusted to 470–500°C, that is, the precipitation order of the ε-Cu phase and the Mo2C phase is interchanged. The remaining components and process parameters are the same as in Example 1.

[0069] Comparative Example 5 The difference between this comparative example and Example 1 is that the S4 pre-heat treatment is omitted, and the process proceeds directly from S3 multi-directional forging to S5 gradient solution treatment. The remaining components and process parameters are the same as in Example 1.

[0070] Performance testing methods Thermal conductivity was tested using the laser flash method, in accordance with GB / T 22588-2008 and ASTM E1461 standards. The samples were processed into circular discs with a diameter of 12.7 mm and a thickness of 2.5 mm. Test temperatures were 25℃, 200℃, 300℃, 400℃, 500℃, 600℃, and 650℃, with three tests performed at each temperature and the average value taken. The thermal conductivity λ was calculated using the formula λ = α × Cp × ρ, where α is the thermal diffusivity, Cp is the specific heat capacity, and ρ is the density.

[0071] Hardness testing was performed using a Rockwell hardness tester, in accordance with GB / T 230.1-2018. Five points were tested on each sample, and the average value was taken.

[0072] The high-temperature yield strength test was conducted in accordance with GB / T 228.2-2015, and the test temperature was 600℃.

[0073] The thermal fatigue life test adopts the cyclic heating-cooling test method. The sample is heated to 650℃ and held for 30 seconds, then cooled to room temperature with water. The number of cycles when visible cracks appear is recorded.

[0074] The content of retained austenite was determined by X-ray diffraction, in accordance with YB / T 5338-2019.

[0075] The microstructure and precipitates were observed using scanning electron microscopy and transmission electron microscopy.

[0076] In XRD phase analysis, the characteristic diffraction peaks of the ε-Cu phase are located at 2θ≈43.3°, while the characteristic diffraction peaks of the Mo2C phase are located at 2θ≈37.5° and 39.8°.

[0077] The thermal conductivity test results of each sample at different temperatures are shown in Table 1: Table 1 Thermal conductivity (W / (m·K)) of each embodiment and comparative example at different temperatures

[0078] Note: The room temperature thermal conductivity of Comparative Example 3 is the nominal value in the product manual (nominal room temperature thermal conductivity 35-37 W / (m·K)), other temperature data were not obtained.

[0079] As shown in Table 1, the room temperature thermal conductivity of Examples 1-3 was 34.8-35.6 W / (m·K), comparable to Comparative Example 3, and significantly higher than 25.0 W / (m·K) of Comparative Example 1 and 28.3 W / (m·K) of Comparative Example 2. In the temperature range of 400-500℃, the thermal conductivity of Examples 1-3 reached a peak of 42.5-45.2 W / (m·K), an increase of approximately 23-30% compared to room temperature. The thermal conductivity of Comparative Example 4 at 500℃ was 37.2 W / (m·K), a decrease of approximately 16.6% compared to Example 1; the thermal conductivity of Comparative Example 5 at 500℃ was 38.5 W / (m·K), a decrease of approximately 13.7% compared to Example 1. At 600℃ and 650℃, the thermal conductivity of Examples 1-3 remained at 41.0-43.6 W / (m·K).

[0080] The test results for hardness, yield strength at 600℃, and thermal fatigue life of each sample are shown in Table 2: Table 2. Hardness, high-temperature yield strength, and thermal fatigue life of each embodiment and comparative example.

[0081] As shown in Table 2, the hardness of Examples 1 to 3 is 48.0 to 49.0 HRC, the yield strength at 600℃ is 810 to 840 MPa, and the thermal fatigue life is 2780 to 2920 cycles, all of which are better than Comparative Example 1.

[0082] The residual austenite content and precipitate characteristic parameters of each sample are shown in Table 3: Table 3. Residual austenite content and precipitate characteristics of each embodiment and comparative example.

[0083] Note: Comparative Example 1 does not contain Cu, therefore no ε-Cu phase precipitates.

[0084] Table 3 shows that the retained austenite content of Examples 1-3 was 10.8-12.5%, the average size of the ε-Cu phase was 4.0-4.5 nm, the average size of the Mo2C phase was 3.5-4.0 nm, and the ε-Cu / Mo2C ratio was 2.5:1-3.0:1. In Comparative Example 2, the average size of the ε-Cu phase was 8.5 nm, the average size of the Mo2C phase was 12.0 nm, and the ε-Cu / Mo2C ratio was 0.8:1. In Comparative Example 4, the average size of the ε-Cu phase was 7.8 nm, the average size of the Mo2C phase was 8.5 nm, and the ε-Cu / Mo2C ratio was 1.2:1. In Comparative Example 5, the average size of the ε-Cu phase was 9.0 nm, the average size of the Mo2C phase was 6.5 nm, and the ε-Cu / Mo2C ratio was 1.5:1. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high thermal conductivity die steel for aluminum alloy die casting, characterized in that, Its chemical composition, by mass percentage, is as follows: The composition is as follows: C 0.20–0.26%, Si 0.05–0.12%, Mn 0.20–0.35%, Cr 0.80–1.20%, Mo 2.0–2.5%, Ni 2.8–3.5%, Al 0.30–0.50%, Cu 0.60–1.20%, V 0.05–0.12%, Co 0.50–1.00%, Nb 0.01–0.03%, RE 0.01–0.05%, with the balance being Fe and unavoidable impurities, wherein the RE is a mixture of rare earth elements with a Ce to La mass ratio of 2:1; P ≤ 0.015%, S ≤ 0.005%.

2. A process for preparing high thermal conductivity die steel, used to prepare the high thermal conductivity die steel for aluminum alloy die casting as described in claim 1, characterized in that, Includes the following steps: S1 Rare Earth Pre-modification Treatment: Mixed rare earth is added at the steel refining temperature, and after electromagnetic stirring, it is allowed to stand and then electroslag remelted. S2 High-temperature homogenization treatment: The steel ingot after electroslag remelting is held at 1220-1250℃ for 20-25 hours, cooled to 850℃ at a cooling rate not exceeding 50℃ / h, and then air-cooled to room temperature; S3 Multi-directional forging: The steel ingot is forged alternately in three directions at 1150~1180℃, and the final forging temperature is not lower than 900℃; S4 Preliminary heat treatment: The forgings are sequentially held at four temperatures of 350℃, 620℃, 780℃ and 880℃ in a stepped heat treatment manner, and then air-cooled to room temperature after heat treatment; S5 Gradient solution treatment: The forging is held at 850-880℃, then held at 940-970℃, then cooled to 900℃, then heated to 1020-1040℃ and held, and finally held at 980-1000℃. S6 Isothermal graded quenching: The solution-treated forgings are held isothermally at 380-420℃, then isothermally at 280-320℃, and finally isothermally at 150-180℃. S7 Cryogenic Treatment: The quenched forgings are kept at -80 to -100°C and then restored to room temperature. S8 Cyclic Phase Transformation Tempering: After cryogenic treatment, the forging is held at 470-500℃ and then cooled to 200℃, then held at 520-550℃ and then cooled to 200℃, and finally held at 490-520℃ and then cooled to room temperature. S9 Stabilization Treatment: Cool the tempered forging to 180°C and hold for a period of time, then cool to room temperature. S10 Temperature Control Response Activation Process: The stabilized forgings are held at 560-590℃ and then cooled.

3. The process for preparing high thermal conductivity mold steel according to claim 2, characterized in that, In S1, the refining temperature is 1550-1580℃, the amount of mixed rare earth added is 0.02-0.05% of the mass of the molten steel, the electromagnetic stirring time is 10-15 minutes, and the settling time is 5-8 minutes.

4. The process for preparing high thermal conductivity mold steel according to claim 2, characterized in that, In S3, the forging ratio in each direction is not less than 2, and the total forging ratio is not less than 6. After forging, the temperature is cooled to 650°C at a cooling rate of not less than 50°C / min, and then transferred to an annealing furnace for cooling.

5. The process for preparing high thermal conductivity mold steel according to claim 2, characterized in that, In S4, the temperature is increased to 350℃ at a heating rate of 40℃ / h and held for 4 hours, then increased to 620℃ at a heating rate of 50℃ / h and held for 6 hours, then increased to 780℃ at a heating rate of 60℃ / h and held for 3 hours, and finally increased to 880℃ at a heating rate of 100℃ / h and held for 2 hours.

6. The process for preparing high thermal conductivity mold steel according to claim 2, characterized in that, In S5, the temperature is maintained at 850-880℃ for 1 hour, then at 940-970℃ for 1.5 hours, then cooled to 900℃ at a cooling rate of 15℃ / min, then maintained at 1020-1040℃ for 1 hour, and finally maintained at 980-1000℃ for 0.5 hours.

7. The process for preparing high thermal conductivity mold steel according to claim 6, characterized in that, In S6, maintain an isothermal temperature of 380–420℃ for 20–30 minutes, then maintain an isothermal temperature of 280–320℃ for 60–90 minutes, and finally maintain an isothermal temperature of 150–180℃ for 30 minutes.

8. The process for preparing high thermal conductivity mold steel according to claim 2, characterized in that, In S7, the holding time is 2-4 hours, and the temperature is restored to room temperature at a heating rate of 20℃ / h; in S8, the temperature is raised to 470-500℃ at a heating rate of 150℃ / h and held for 2.5 hours, then cooled to 200℃ at a cooling rate of 60℃ / h, then raised to 520-550℃ at a heating rate of 120℃ / h and held for 2 hours, then cooled to 200℃ at a cooling rate of 50℃ / h, and finally raised to 490-520℃ at a heating rate of 100℃ / h and held for 1.5 hours, then cooled to room temperature at a cooling rate of 30℃ / h.

9. The process for preparing high thermal conductivity mold steel according to claim 2, characterized in that, In S9, the temperature is cooled to 180℃ at a cooling rate of 8℃ / min and held for 6 hours, then cooled to room temperature at a cooling rate of 15℃ / min; in S10, the temperature is held at 560~590℃ for 2 hours, then cooled to 450℃ at a cooling rate of 200℃ / h, then cooled to room temperature at a cooling rate of 50℃ / h.

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