Die steel for injection mold and method for manufacturing the same

Abstract

The application relates to the technical field of alloy steel manufacturing, and particularly discloses a die steel for injection molds and a manufacturing method thereof.In the application, the composition of the die steel comprises, in percentage by weight, C 0.25%-0.35%, Mo 1.80%-2.40%, Si 0.08%-0.30%, Mn 0.08%-0.30%, Cr 0.08%-0.20%, V <=0.02%, P <=0.010%, and S <=0.005%, and the rest is Fe and inevitable impurity elements.The application realizes the collaborative improvement of the thermal conductivity and the mechanical properties by the precise matching of the low-alloy design and the multi-stage heat treatment, the systematic optimization of annealing, pre-hardening, tempering and other heat treatments, and the thermal conductivity is significantly better than that of the commonly used steel grades 1.2083, P20 and H13 in the industry, thereby providing key technical support for the development of the injection molding industry in the direction of high efficiency and precision.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of alloy steel manufacturing, in particular to a mold steel for injection mold and a preparation method thereof. BACKGROUND

[0002] The production principle of injection mold is to inject molten plastic particles into a steel cavity precisely processed according to the product shape under the action of high temperature and high pressure. After the plastic is cooled and solidified in the cavity, the mold is opened to obtain the target shaped product. In the entire molding cycle, the cooling process (from the end of injection to the opening stage) is crucial. The mold cooling efficiency directly determines the plastic solidification speed, and an efficient cooling system can shorten the molding cycle, improve the unit time capacity, and reduce the defects such as product deformation and shrinkage caused by uneven cooling, which has a decisive significance for the improvement of production efficiency and product quality.

[0003] In the process field of injection molding polypropylene (PP) products, the cavity and core materials are usually pre-hardened mold steels. At present, the mold steel materials widely used in the industry include imported grades 1.2083, 1.2311, 1.2738 and 1.2738HH, and domestic SWP20, SW718H, etc. However, the thermal conductivity of the above-mentioned steels is only 29-34 W / (m·K) at room temperature, and with the continuous improvement of the market's requirements for injection molding efficiency and product precision, this heat conduction performance has been difficult to meet the increasingly stringent process requirements.

[0004] For the local cooling difficult area in the mold due to the special structure of the product, the existing technology often uses high thermal conductivity copper alloy materials such as Ampco83, MolmaxHH, PW300 and Jumpole70T. Although the thermal conductivity of such copper alloys is significantly better than traditional mold steels, they generally have poor wear resistance or insufficient toughness or obvious defects in comprehensive performance, which leads to surface damage, fracture failure and other situations in the actual use of the mold, making it difficult to meet the long-term stable demand of industrial batch production. SUMMARY

[0005] In view of the technical bottleneck of insufficient heat conduction performance of the existing mold steel for injection molding, and the problem of poor comprehensive mechanical properties of high thermal conductivity alternative materials, the present application provides a mold steel for injection mold and a preparation method thereof through alloy composition design and process optimization, which can prepare a new type of mold steel for injection molding with high thermal conductivity and excellent mechanical properties, providing a new solution and technical path for the technical upgrading and market demand satisfaction of the injection mold field.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0007] In a first aspect, the present application provides a mold steel for injection mold, which comprises the following chemical components in percentage by weight: C 0.25%-0.35%, Mo 1.80%-2.40%, Si 0.08%-0.30%, Mn 0.08%-0.30%, Cr 0.08%-0.20%, V ≤0.02%, P ≤0.010%, S ≤0.005%; and the rest is Fe and inevitable impurity elements.

[0008] In the alloying idea, the mold steel for injection mold provided by the present application is characterized by simple alloying, mainly with C and Mo two elements; secondly, keeping low Si, Mn and Cr elements; through reasonable alloying component design, the mold steel provided by the present application not only saves economic cost, but also has high thermal conductivity (thermal conductivity coefficient reaches 50-54.5 W / (m*K)) and excellent mechanical properties (hardness 35-39 HRC, impact toughness ≥49 J / cm²).

[0009] Thermal conductivity is one of the most important thermal physical properties of hot work die steel, and the thermal conductivity also contains two physical properties of heat capacity and thermal diffusivity. The mold made of high thermal conductivity mold steel can have faster cooling rate and shorter thermal cycle time, which accelerates the heat diffusion from the cavity to the surface of the mold, so as to manufacture higher strength, thinner parts, design parts with more complex geometry and reduce production cost. High thermal conductivity mold steel can reduce the temperature gradient from the inside to the surface of the mold, reduce the stress amplitude change caused by thermal cycle, so as to avoid stress concentration of the mold and prevent the mold from being locally deformed and cracked to fail. Although the overall cooling capacity of the mold can be improved by appropriate mold optimization design and increasing cooling channels, the optimization of the cooling channel system requires high related processing requirements, which will complicate the mold design and greatly increase the cost of mold maintenance.

[0010] Theoretically, the thermal conductivity of ferrite in the organization of the steel is the highest, about 71-80 W / (m*K), the thermal conductivity of tempered martensite is about 35 W / (m*K), and the thermal conductivity of cementite is the lowest, about 7 W / (m*K), so the thermal conductivity of the mold steel can be improved through the design of alloying components, but how to adjust the content of each element in the mold steel to improve its thermal conductivity without reducing the mechanical properties of the mold steel, especially the high-temperature thermal conductivity, is the key to develop new hot work die steel.

[0011] The roles and amounts of important elements in the present application are as follows:

[0012] C element: 0.25%~0.35%. Carbon is one of the main chemical elements of high thermal strength die steel, part of carbon enters the matrix to play a solid solution strengthening role, another part is to form chromium carbide, molybdenum carbide, vanadium carbide and various types of carbide, some of the alloy carbides can diffuse and precipitate on the quenched martensite matrix during tempering to produce secondary hardening phenomenon. Carbon element is an important element affecting the composition segregation of steel and the uniformity of the organization of steel. In terms of thermal conductivity: the damage of carbon element in the form of second phase to the heat conduction capacity of steel is smaller than that in the form of solid solution in the matrix, it can make strong carbide and Mn weak carbide enter the carbide from the matrix, thereby improving the thermal conductivity. Therefore, if the carbon content is higher than the upper limit of the composition design, it will lead to the formation of too much carbide and the segregation of the organization, which will affect the impact toughness performance index of the steel, especially cause the serious unevenness of the liquid precipitation carbide of the steel, so that the impact toughness of the steel is reduced; but if the carbon element is lower than the design range of the composition, it will also cause the deviation of the equivalent of the combination of carbon element and other alloy elements to form carbide, and the stable molybdenum carbide, tungsten carbide and various types of carbide complex effect cannot be effectively formed, which will affect the hardness, impact toughness, wear resistance and high temperature performance of the steel. In short, the present application precisely controls the content of carbon element, on the basis of ensuring the martensite transformation strengthening, avoids the hindering of excessive carbide formation to heat conduction, and realizes the balance of strength and thermal conductivity.

[0013] Mo element: 1.80%~2.40%. Molybdenum is a strong carbide-forming element, and is one of the important chemical elements in the design composition. It can reduce the activity of carbon in steel, and its carbide is stable and not easy to grow, so it can play a role in refining the grain, thereby greatly improving the tempering stability of the steel. Molybdenum can greatly delay the pearlite transformation, and the influence on bainite transformation is small, so the steel can effectively improve the hardenability of the steel. Molybdenum can effectively inhibit the segregation of harmful elements in the steel, and is an effective element for eliminating or reducing the high-temperature temper brittleness of the steel. Molybdenum can strongly improve the interatomic binding force of the solid solution, so it can improve the thermal strength of the steel. It can maintain sufficient strength and anti-creep ability (long-term stress under high temperature, deformation is called creep) at high temperature. It can improve the red hardness in tool steel. The addition of molybdenum can improve the stability of austenite and the hardenability of the steel. During the tempering process of the steel, molybdenum can combine with carbon to form a large number of M2C alloy carbides. This kind of carbide is stable and dispersed, and can have a good secondary hardening effect. The addition amount of molybdenum is controlled in this range, so that more M2C alloy carbides are obtained during the tempering process of the steel, and a larger secondary strengthening effect is produced, which plays an important role in improving the hardness and impact toughness of the steel. When the content of molybdenum exceeds a certain range, element segregation, ferrite delta phase or other brittle phases may occur, which reduces the impact toughness. In summary, by controlling the Mo content to be 1.80%~2.40%, fine and dispersed molybdenum carbides (such as Mo2C) are formed, which effectively hinder dislocation movement and improve the high-temperature strength and wear resistance of the material. At the same time, Mo is solid-solved in the matrix, which optimizes the electron migration path and has less negative impact on thermal conductivity.

[0014] Si element: 0.08%~0.30%. The addition of elements with large differences in outer electron structure can affect the thermal conductivity of the steel. Since the outer electron structure of Si is quite different from that of Fe, the content of Si in the steel needs to be controlled. Controlling Si can reduce the amount of inclusions, make the macrostructure more uniform, refine the dendritic crystal of the microstructure, and reduce the composition supercooling at the solidification interface during solidification, thereby increasing the plasticity and toughness of the steel. Silicon is a very effective element for substitutional solid solution strengthening with ferrite, but it can reduce the toughness and plasticity of the steel to some extent. The addition of Si can improve the hardenability of the steel. Si also helps to improve the dispersity of special carbides precipitated during high-temperature tempering, which can increase the secondary hardening peak, so Si is an effective element for improving the strength of the matrix and the tempering resistance. The role of silicon element is to slow down the decomposition of martensite during the tempering process, and can effectively hinder the decomposition of martensite after the transformation from austenite to martensite.

[0015] Mn element: 0.08%~0.30%. Manganese is a good deoxidizer and desulfurizer in the steelmaking process. Although manganese is a weak carbide forming element, it cannot form carbide strengthening, but the addition of a certain amount of manganese element can promote the decomposition of cementite and delay the precipitation and growth of carbide, which is beneficial to the thermal stability of the steel. Manganese has a strong affinity with sulfur, which can avoid the formation of low-melting-point sulfide FeS on the grain boundary, and exist as high-melting-point MnS with certain plasticity, which can prevent the thermal embrittlement caused by FeS and eliminate the harmful effects of sulfur, thereby improving the hot working properties of the steel. Manganese dissolved in austenite can strongly increase the hardenability of the steel, while strongly reducing the Ms point of the steel. Mn has a solid solution strengthening effect, thereby increasing the strength and hardness of ferrite and austenite, although its solid solution strengthening effect is not as good as that of carbon, phosphorus and silicon, but it has little effect on the ductility of the steel. In addition, manganese element can increase and stabilize the content of residual austenite in the steel, which can improve the toughness and thermal fatigue resistance of the steel. However, too high manganese content will increase the brittleness of the steel, weaken the corrosion resistance of the steel, and reduce the thermal conductivity and welding performance.

[0016] Cr element: 0.08%~0.20%. Chromium is a medium-strong carbide forming element, part of the chromium in the steel replaces to form alloy cementite, improves its stability; part of it dissolves in ferrite, produces solid solution strengthening, increases the strength and hardness of ferrite. Most of the existing hot work die steels add chromium element. Considering that the diffusion speed of chromium in austenite is relatively small, and it hinders the diffusion of carbon, thereby improving the stability of austenite and the hardenability of the steel of the present application. When the content of chromium is low, chromium alloy cementite (Fe, Cr) 3C may be formed, and with the increase of chromium content, the carbide in the steel gradually changes to Cr7C3 and Cr 23 C6. Cr7C3 and Cr 23 C6 can be dispersedly distributed during tempering, which can improve the tempering resistance of the steel, and can make the steel have certain red hardness, and improve the thermal strength of the steel. However, when the tempering temperature is higher than 600℃, the chromium carbide will quickly gather and coarsen, which makes the tempering stability of the steel poor. Therefore, the present design mainly uses the carbide of Mo to replace the effect of chromium carbide, which not only has the same effect, but also reduces the influence of Cr on thermal conductivity and reduces the alloy cost of the steel.

[0017] In the present application, by limiting the content of trace elements such as Si, Mn and Cr, the lattice distortion caused by solid solution strengthening is reduced, the continuity of the matrix structure is maintained, and the efficient heat conduction path is ensured.

[0018] In a second aspect, the present application provides a preparation method of the mold steel for injection mold, which comprises the following steps: smelting, forging according to the chemical components in the above weight percentage, obtaining a forged piece, annealing, pre-hardening and tempering to obtain the mold steel for injection mold.

[0019] After the forging is kept at 830 DEG C ~ 880 DEG C for 15h~20h and then is dividedly cooled, an annealed forging is obtained;

[0020] After the annealed forging is heated to 980 DEG C ~ 1000 DEG C and kept for 4h~6h and then is cooled to 20 DEG C ~ 30 DEG C, a pre-hardened forging is obtained;

[0021] After the pre-hardened forging is tempered, a die steel for injection mold is obtained.

[0022] The present application breaks through the traditional contradiction between thermal conductivity and mechanical properties by synergistic regulation of alloy composition design and multi-stage heat treatment process, optimizes the grain boundary structure at the micro level, and realizes the synchronous improvement of heat conduction efficiency and mechanical properties. In addition, the preparation method is highly compatible with the existing die steel production process, does not require special equipment, and is easy to scale up.

[0023] In the present application, the annealing process uses long-time heat preservation (15h~20h) at 830 DEG C ~ 880 DEG C to fully release the forging stress and uniformly diffuse the alloying elements, laying a good foundation for subsequent heat treatment. After annealing, the forging is dividedly cooled to inhibit the formation of coarse carbides, retain fine and dispersed MC type carbides, strengthen the matrix, and maintain high heat conduction channel efficiency. During pre-hardening, high-temperature austenitizing at 980 DEG C ~ 1000 DEG C is followed by rapid quenching to form uniform and fine martensite structure, thereby improving the hardness and strength of the forging.

[0024] Preferably, the forging specifically comprises the following steps:

[0025] The electrode billet obtained by steelmaking is heated to 830 DEG C ~ 880 DEG C, kept for 15h~20h, and then cooled to obtain an annealed electrode billet; the annealed electrode billet has a certain thickness of an oxide layer on the surface, and the surface of the electrode billet is polished by a roll grinder to avoid the increase of gaseous oxygen in the electroslag process.

[0026] The annealed electrode billet is subjected to electroslag remelting in an inert gas atmosphere at 45V~48V and 21000A~25000A to obtain an electroslag ingot. In this process, the content of gas and inclusions can be reduced by electroslag remelting in an inert atmosphere, and an electroslag ingot with uniform composition, dense structure and low segregation is obtained.

[0027] The electroslag ingot is heated to 1230 DEG C ~ 1250 DEG C and kept for 20h~25h to homogenize the composition, reduce element segregation, and improve the as-cast solidification structure; the furnace is cooled to 1000 DEG C ~ 1050 DEG C, then heated to 1180 DEG C ~ 1200 DEG C (initial forging temperature), kept for 4h~6h, and then subjected to multi-directional forging to obtain a forging.

[0028] Preferably, the cooling is furnace cooling to 450 DEG C ~ 500 DEG C followed by air cooling.

[0029] Preferably, the melting rate of the electroslag remelting is 11.0 kg / min~12.5 kg / min.

[0030] Preferably, the final forging temperature of the multi-directional forging is 850℃~900℃, and the forging ratio is 10~14.

[0031] Preferably, the segmented cooling is specifically cooling to 450℃~500℃ at a speed of 20℃ / h~50℃ / h and then air cooling to room temperature after discharging.

[0032] Preferably, the tempering is specifically heating the pre-hardened forging to 520℃~560℃ and holding for 9h~11h, and then cooling.

[0033] Preferably, the tempering is performed twice.

[0034] To sum up, the present application realizes the collaborative improvement of thermal conductivity and mechanical properties by the precise matching of low alloy design and multi-stage heat treatment, especially the thermal conductivity is significantly higher than the commonly used steel grades 1.2083, P20 and H13 in the industry, which can effectively solve the technical problems of existing mold materials and provide key technical support for the development of injection molding industry in the direction of high efficiency and precision. BRIEF DESCRIPTION OF DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0036] Figure 1 It is a metallographic phase diagram of the annealed forging in the embodiment 1 of the present application, wherein Fig. A) is a metallographic phase diagram enlarged 100 times, and Fig. B) is a metallographic phase diagram enlarged 500 times;

[0037] Figure 2 It is a metallographic phase diagram of the mold steel for injection mold in the embodiment 1 of the present application, wherein Fig. A) is a metallographic phase diagram enlarged 100 times, and Fig. B) is a metallographic phase diagram enlarged 500 times;

[0038] Figure 3 It is a metallographic phase diagram of the annealed forging in the embodiment 2 of the present application, wherein Fig. A) is a metallographic phase diagram enlarged 100 times, and Fig. B) is a metallographic phase diagram enlarged 500 times;

[0039] Figure 4The microstructure of the mold steel for injection mold in the embodiment 2 of the present application; wherein, Fig. A) is the microstructure enlarged 100 times, Fig. B) is the microstructure enlarged 500 times;

[0040] Figure 5 The microstructure of the forged piece in annealing state in the embodiment 3 of the present application; wherein, Fig. A) is the microstructure enlarged 100 times, Fig. B) is the microstructure enlarged 500 times;

[0041] Figure 6 The microstructure of the mold steel for injection mold in the embodiment 3 of the present application; wherein, Fig. A) is the microstructure enlarged 100 times, Fig. B) is the microstructure enlarged 500 times;

[0042] Figure 7 The microstructure of the forged piece in annealing state in the comparative example 1 of the present application; wherein, Fig. A) is the microstructure enlarged 100 times, Fig. B) is the microstructure enlarged 500 times;

[0043] Figure 8 The appearance result of the forged piece in annealing state prepared in the comparative example 2 of the present application after being placed for 2 days;

[0044] Figure 9 The microstructure of the mold steel for injection mold in the comparative example 3 of the present application; wherein, Fig. A) is the microstructure enlarged 100 times, Fig. B) is the microstructure enlarged 500 times;

[0045] Figure 10 The comparison chart of the volume wear rate of the mold steel for injection mold provided by the present application and the common plastic mold steel in the industry in the effect example 2 of the present application;

[0046] Figure 11 The comparison chart of the thermal conductivity of the mold steel for injection mold provided by the present application and the common plastic mold steel in the industry in the effect example 2 of the present application. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0048] Embodiment 1

[0049] The embodiment of the present application provides a preparation method of the mold steel for injection mold, and the preparation method comprises the following steps:

[0050] (a) electric furnace smelting

[0051] The preparation raw material is smelted at 1550-1750 DEG C through a traditional electric furnace smelting process, and then is refined outside the LF furnace and is vacuum degassed by VD to improve the purity of the molten steel, and is poured into an electrode blank with a diameter of 680 mm. The preparation raw material comprises the following components by weight percentage: C 0.28%, Mo 2.0%, Si 0.1%, Mn 0.12%, Cr 0.10%, V 0.01%, and the rest is Fe and inevitable impurity elements, and the impurities are P≤0.005% and S≤0.002%.

[0052] (b) electrode blank annealing

[0053] The electrode blank is heated to 850 DEG C at a heating rate of 80 DEG C / min, and is cooled to 500±5 DEG C after holding for 15 h, and then is taken out of the furnace and is air-cooled to room temperature, so that internal stress is effectively removed, the electrode blank is not broken in the subsequent electroslag process, and the internal quality is affected, and the annealed electrode blank is obtained. The surface of the annealed electrode blank is polished by a roll grinder, so that the surface scum and the oxide layer are effectively removed, and the purity of the material is ensured.

[0054] (c) electroslag remelting

[0055] The electroslag remelting is performed in an argon atmosphere, the mold has a diameter of 920 mm, the electroslag remelting voltage is 45-48 V, the current is 21000-25000 A, and the melting rate is 11.0-12.5 kg / min, so that the content of gas and inclusions is reduced, and an electroslag ingot with uniform composition, compact structure and low segregation is obtained, and then the electroslag ingot is heated in a red heating furnace.

[0056] (d) high-temperature diffusion

[0057] The electroslag ingot is heated to 1250 DEG C and is held for 20 h, so that the composition of the electroslag ingot is homogenized, the segregation of the elements is reduced, and the as-cast solidification structure is improved, and then the electroslag ingot is cooled to 1000-1050 DEG C, so that the temperature of the core of the electroslag ingot is reduced to below 1180 DEG C, and the temperature of the core is not too high in the subsequent forging process, and internal cracks are avoided.

[0058] (e) forging

[0059] The electroslag ingot is heated to 1180 DEG C again, is held for 6 h, and is subjected to multi-directional forging, and the final forging temperature is 900 DEG C, and the forging ratio is 12, so that a forged piece with a size of 240*830*3000 (mm) is obtained.

[0060] (f) annealing

[0061] The forged piece is heated to 850 DEG C, is held for 15 h, and is cooled to 450±5 DEG C at a rate of 30 DEG C / h, and then is taken out of the furnace and is air-cooled to room temperature, so that an annealed forged piece is obtained. The metallographic pattern of the annealed forged piece is shown in Figure 1The metallographic chart of the magnification of 100 times is shown in Figure A, and the metallographic chart of the magnification of 500 times is shown in Figure B.

[0062] (g) pre-hardening

[0063] The annealed forging is heated to 980 DEG C in the furnace, and then rapidly air-cooled to room temperature after 5h of heat preservation, and tempered at 520 DEG C for 2 times, each time for 10h of heat preservation, to obtain the mold steel for injection mold. The metallographic chart of the mold steel is shown in Figure Figure 2 ; wherein the metallographic chart of the magnification of 100 times is shown in Figure A, and the metallographic chart of the magnification of 500 times is shown in Figure B.

[0064] It can be known from the above table that the annealed forging and the mold steel for injection mold prepared by the method have the following advantages: Figures 1-2 The ferrite and pearlite in the annealed forging and the mold steel for injection mold are uniformly distributed, the irregularly shaped alloy carbides are uniformly distributed, there is no segregation strip structure, and the overall uniformity is good.

[0065] Example 2

[0066] The preparation method of the mold steel for injection mold provided by the embodiment of the application comprises the following steps:

[0067] (a) electric furnace smelting

[0068] The preparation raw material is smelted at 1550 DEG C to 1750 DEG C through a traditional electric furnace smelting process, and then the purity of the molten steel is improved through LF furnace external refining and VD vacuum degassing, and the electrode billet with a specification of φ680mm is poured. The preparation raw material comprises the following components in percentage by weight: C 0.3%, Mo 2.2%, Si 0.1%, Mn 0.10%, Cr 0.08%, V 0.01%, and the rest is Fe and inevitable impurity elements, and the impurities are P≤0.005% and S≤0.002%.

[0069] (b) electrode billet annealing

[0070] The electrode billet is heated to 830 DEG C in the furnace at a heating rate of 80 DEG C / min, and then cooled to 480±5 DEG C in the furnace after 20h of heat preservation, and then air-cooled to room temperature after being taken out of the furnace, so that the internal stress is effectively removed, the electrode is prevented from being broken in the subsequent electroslag process, and the internal quality is affected, and the electrode billet after annealing is obtained. The surface of the electrode billet is polished by a roll grinder, so that the surface dross and the oxidation layer are effectively removed, and the purity of the material is ensured.

[0071] (c) electroslag remelting

[0072] The crystallizer specification is φ920mm, the electroslag remelting voltage is 45V-48V, the current is 21000A-25000A, the melting speed is 11.0kg / min-12.5kg / min, the content of gas and inclusion can be reduced by electroslag remelting under the protection of argon atmosphere, and the electroslag ingot with uniform composition, compact structure and low segregation is obtained, and then the red delivery heating furnace is heated.

[0073] (d) high temperature diffusion

[0074] The above electroslag ingot is heated to 1230℃ and kept for 25h, so that the composition is homogenized, the element segregation is reduced, and the as-cast solidification structure is improved, and then the furnace is cooled to 1000℃-1050℃, so that the temperature of the core of the electroslag ingot is reduced to below 1180℃, and the high temperature of the core in the forging process is avoided, so that the internal crack is caused.

[0075] (e) forging

[0076] The above electroslag ingot is heated to 1200℃ again, kept for 4.5h, and then subjected to multi-directional forging, the final forging temperature is 850℃, the forging ratio is 12, and the forged piece with the specification of 240*830*2500(mm) is obtained.

[0077] (f) annealing

[0078] The above forged piece is heated to 830℃ in the furnace, kept for 20h, and then cooled to 480±5℃ at the speed of 50℃ / h, and then taken out of the furnace and air-cooled to room temperature, so that the annealed forged piece is obtained. The metallographic graph of the annealed forged piece is shown in Figure 3 ; wherein, the metallographic graph with 100 times magnification is shown in Fig. A), and the metallographic graph with 500 times magnification is shown in Fig. B).

[0079] (g) pre-hardening

[0080] The above annealed forged piece is heated to 990℃ in the furnace, kept for 6h, and then rapidly air-cooled to room temperature, tempered at 520℃ for 2 times, each time kept for 11h, so that the mold steel for injection mold is obtained. The metallographic graph of the mold steel for injection mold is shown in Figure 4 ; wherein, the metallographic graph with 100 times magnification is shown in Fig. A), and the metallographic graph with 500 times magnification is shown in Fig. B).

[0081] As known from Figures 3-4 , the annealed forged piece and the mold steel for injection mold prepared by the method of the present application comprise uniformly distributed ferrite and pearlite structure, and irregularly shaped alloy carbide, and there is no segregation band structure.

[0082] Example 3

[0083] The present application provides a preparation method of the mold steel for injection mold, and the preparation method comprises the following steps:

[0084] (a) electro-furnace smelting

[0085] The preparation raw material is smelted at 1550-1750°C through a traditional electric furnace smelting process, and then is refined outside the LF furnace and is vacuum degassed in the VD to improve the purity of the molten steel, and is poured into an electrode blank with a diameter of 680 mm. The preparation raw material comprises the following components by weight percentage: C 0.32%, Mo 2.35%, Si 0.15%, Mn 0.10%, Cr 0.1%, V 0.01%, and the rest is Fe and inevitable impurity elements, and the impurities are P≤0.005% and S≤0.002%.

[0086] (b) Annealing of the electrode blank

[0087] The electrode blank is heated to 880°C at a heating rate of 100°C / min, and is cooled to 460±5°C after holding for 15 h, and then is taken out of the furnace and is air-cooled to room temperature, to obtain the annealed electrode blank. The surface of the annealed electrode blank is polished by a roll grinder to effectively remove the surface dross and the oxide layer, and to ensure the purity of the material.

[0088] (c) Electroslag remelting

[0089] The crystallizer has a diameter of 920 mm, the electroslag remelting voltage is 45-48 V, the current is 21000-25000 A, and the melting rate is 11.0-12.5 kg / min. The electroslag remelting under the protection of argon atmosphere can reduce the content of gas and inclusions, and can obtain an electroslag ingot with uniform composition, dense structure and low segregation, which is then heated in a red heating furnace.

[0090] (d) High-temperature diffusion

[0091] The electroslag ingot is heated to 1250°C and is held for 22 h, so that the composition of the electroslag ingot is homogenized, the segregation of the elements is reduced, and the as-cast solidification structure is improved. Then, the electroslag ingot is cooled to 1000-1050°C, so that the temperature of the core of the electroslag ingot is reduced to below 1180°C, and the high temperature of the core during the forging process is avoided, which can cause internal cracks.

[0092] (e) Forging

[0093] The electroslag ingot is heated to 1190°C again, and is held for 5 h before being subjected to multi-directional forging. The final forging temperature is 900°C, and the forging ratio is 12, to obtain a forged piece with a size of 240*830*2500 (mm).

[0094] (f) Annealing

[0095] The forged piece is heated to 880°C in the furnace, is held for 15 h, and then is cooled to 500±5°C at a rate of 40°C / h, and is taken out of the furnace and is air-cooled to room temperature, to obtain an annealed forged piece. The metallographic pattern of the annealed forged piece is shown in Fig. 4. Figure 5As shown in the figure; wherein the metallographic microstructure with 100 times magnification is shown in Figure A), and the metallographic microstructure with 500 times magnification is shown in Figure B).

[0096] (g) pre-hardening

[0097] The annealed forging is heated to 1000 DEG C in the furnace, and then cooled to room temperature rapidly after holding for 4 hours, and tempered at 560 DEG C twice, each time holding for 10 hours, to obtain the mold steel for injection mold. Figure 6 As shown in the figure; wherein the metallographic microstructure with 100 times magnification is shown in Figure A), and the metallographic microstructure with 500 times magnification is shown in Figure B).

[0098] As shown in the figure; wherein the metallographic microstructure with 100 times magnification is shown in Figure A), and the metallographic microstructure with 500 times magnification is shown in Figure B). Figures 5-6 It can be seen that the annealed forging and the mold steel for injection mold prepared by the method have typical homogeneous microstructure, and the ferrite and pearlite are uniformly distributed, and there is no composition segregation band in the longitudinal section.

[0099] Comparative Example 1

[0100] The comparative example provides a preparation method of the mold steel for injection mold, which is basically the same as the preparation method provided in the example 1, and the difference is only that in the step (a) of electric furnace smelting, the preparation raw material comprises the following components with the following weight percentages: C 0.28%, Mo 2.6%, Si 0.08%, Mn 0.10%, Cr 0.1%, V 0.01%, and the rest is Fe and inevitable impurity elements, and the impurities are P≤0.005%, S≤0.002%; the rest of the steps and parameters are the same as those in the example 1, and finally the mold steel for injection mold is prepared.

[0101] As shown in the figure; wherein the metallographic microstructure with 100 times magnification is shown in Figure A), and the metallographic microstructure with 500 times magnification is shown in Figure B). Figure 7 As shown in the figure; wherein the metallographic microstructure with 100 times magnification is shown in Figure A), and the metallographic microstructure with 500 times magnification is shown in Figure B). Figure 7 It can be seen that there is obvious banded structure, but the carbide distribution is not uniform due to the high Mo content, which will affect the uniformity of the material structure and further affect the impact toughness of the material.

[0102] Comparative Example 2

[0103] The comparative example provides a preparation method of a mold steel for injection mold, which is basically the same as the preparation method provided in Example 2, and the only difference is that in step (f) annealing, the obtained forging is heated to 800°C in the furnace, and then cooled to 480±5°C at a speed of 50°C / h, and then taken out of the furnace and air-cooled to room temperature to obtain an annealed forging; (2) no pre-hardening treatment is performed in step (g); the remaining steps and parameters are the same as those in Example 2, and finally an annealed forging is prepared. It is planned to pre-harden the annealed forging into a mold steel for injection mold, but it is found that the annealed forging appears stress cracks after being placed for 2 days after being taken out of the furnace under this process, and the specific picture is shown in Figure 8 The reason may be that the residual stress is high after annealing at 800°C, and cracks occur during storage, which indicates that the annealing method of Comparative Example 2 is not suitable for the steel of the present application.

[0104] Comparative Example 3

[0105] The comparative example provides a preparation method of a mold steel for injection mold, which is basically the same as the preparation method provided in Example 2, and the only difference is that in step (f) annealing, the obtained forging is heated to 800°C in the furnace, and then cooled to 480±5°C at a speed of 50°C / h, and then taken out of the furnace and air-cooled to room temperature to obtain an annealed forging; (2) no pre-hardening treatment is performed in step (g); the remaining steps and parameters are the same as those in Example 2, and finally an annealed forging is prepared. It is planned to pre-harden the annealed forging into a mold steel for injection mold, but it is found that the annealed forging appears stress cracks after being placed for 2 days after being taken out of the furnace under this process, and the specific picture is shown in Figure 9 The metallographic chart of the mold steel for injection mold is shown in

[0106] It can be seen from Figure 9 that the metallographic sample finds that the local carbide is not completely remelted and is not uniformly redistributed, which will affect the uniformity of the structure and may reduce the impact toughness of the material.

[0107] Effect Example 1

[0108] This effect example investigates the hardness, impact performance and thermal conductivity of the mold steel for injection mold prepared in Examples 1-3, Comparative Examples 1 and 3. Because the performance of the product prepared in Comparative Example 2 is obviously not suitable for this steel, the determination of related indicators is not performed. The determination methods for evaluating the hardness, impact performance and thermal conductivity in the present application are as follows:

[0109] (1) The hardness of different mold steels for injection mold is tested according to the method recorded in GB / T 230.1-2018 “Metallic Materials Rockwell Hardness Test Part 1: Test Method”;

[0110] (2) The transverse KU2 impact performance of the center position of the cross section of different mold steels for injection mold is determined according to the method recorded in GB / T 229 “Metallic Materials Charpy Pendulum Impact Test Method”;

[0111] (3) The thermal conductivity of different injection mold steels at 20℃, 400℃ and 600℃ was tested according to the method described in ASTM E1461-11 "Standard Test Method for Thermal Diffusivity by the Flash Method".

[0112] The results of the test of hardness, impact toughness and thermal conductivity of different injection mold steels are shown in Table 1.

[0113] Table 1

[0114]

[0115] As can be seen from the data in Table 1, while maintaining a certain level of hardness, the impact energy and thermal conductivity of the mold steel for injection molds prepared in the embodiments of the present invention are significantly higher than those of the comparative example, indicating that the product provided by the present invention has superior impact toughness and thermal conductivity, and better overall performance.

[0116] Example 2

[0117] This invention uses the injection mold steel prepared in Example 1 as an example, and compares its wear resistance and thermal conductivity with commonly used plastic mold steels in the industry, such as 1.2083, P20, and H13. The comparison charts of the volumetric wear rate and thermal conductivity results of the injection mold steel prepared in Example 1 and commonly used plastic mold steels in the industry are shown below. Figures 10-11 As shown.

[0118] The method for determining wear resistance in this invention is as follows: Samples of 10*10*10mm size are prepared from injection molds obtained from 1.2083, P20, H13, and Example 1 using mold steel. After polishing, the materials are made to have the same surface roughness. The volumetric wear rate of the samples is tested using a GF-1 type reciprocating friction and wear testing machine. Specifically, a 6mm diameter GCr15 steel ball is used as the abrasive material, and the sample is reciprocated under a load of 40N at a frequency of 3Hz, a wear trajectory length of 5mm, and a friction time of 30min. Finally, the wear volume is measured using an MT-500 material surface wear track measuring instrument.

[0119] Depend on Figure 10 It can be seen that the wear resistance of the mold steel for injection molds provided by the present invention is comparable to that of 1.2083 and H13, but weaker than that of P20.

[0120] Depend on Figure 11 It can be seen that the thermal conductivity of the injection mold steel provided by the present invention in each temperature range is significantly higher than that of commonly used plastic mold steels in the industry, such as 1.2083, P20, and H13.

[0121] As can be known from the above, the product provided by the application has obvious improvement in thermal conductivity under the condition of having equivalent wear resistance as the steel commonly used in the industry, and further has faster cooling rate and shorter thermal cycle time, and the application provides key technical support for the development of injection molding industry in the direction of high efficiency and precision.

[0122] The above merely describes preferred embodiments of the application, but should not be used to limit the application, and any modification, equivalent replacement or improvement made within the spirit and principle of the application should be included in the protection scope of the application.

Claims

1. A mold steel for injection molds, characterized by, Chemical components including the following weight percentages: C 0.25%~0.35%, Mo 1.80%~2.40%, Si 0.08%~0.30%, Mn 0.08%~0.30%, Cr 0.08%~0.20%, V ≤0.02%, P ≤0.010%, S ≤0.005%; the rest is Fe and inevitable impurity elements; The preparation method of the injection mold mold steel comprises the following steps: Steeling and forging are performed according to preset chemical components by weight percentage, and a forged piece is obtained; The forged piece is subjected to segmented cooling at 830-880 DEG C for 15-20 hours, and an annealed forged piece is obtained; The annealed forged piece is heated to 980-1000 DEG C for 4-6 hours, and then cooled to 20-30 DEG C, and a pre-hardened forged piece is obtained; The pre-hardened forged piece is tempered, and an injection mold mold steel is obtained; The segmented cooling is specifically as follows: the forged piece is cooled to 450-500 DEG C at a speed of 20-50 DEG C / h, and then air-cooled after being taken out of the furnace; The tempering is specifically as follows: the pre-hardened forged piece is heated to 520-560 DEG C and kept for 9-11 hours, and then cooled.

2. The mold steel for injection molds according to claim 1, characterized by, The forging specifically comprises the following steps: The electrode blank obtained by steeling is heated to 830-880 DEG C, kept for 15-20 hours, and then cooled, and an annealed electrode blank is obtained; The annealed electrode blank is subjected to electroslag remelting in an inert gas atmosphere at 45-48 V and 21000-25000 A, and an electroslag ingot is obtained; The electroslag ingot is heated to 1230-1250 DEG C, kept for 20-25 hours, cooled to 1000-1050 DEG C in the furnace, then heated to 1180-1200 DEG C, kept for 4-6 hours, and subjected to multi-directional forging, and the forged piece is obtained.

3. The mold steel for injection molds according to claim 2, characterized by, The cooling is furnace cooling to 450-500 DEG C, and then air-cooled.

4. The mold steel for injection molds according to claim 2, characterized by, The melting rate of the electroslag remelting is 11.0-12.5 kg / min.

5. The mold steel for injection molds according to claim 2, characterized by, The final forging temperature of the multi-directional forging is 850-900 DEG C, and the forging ratio is 10-14.

6. The mold steel for injection molds according to claim 1, characterized by, The tempering is performed twice.

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