Tool steel and preparation method thereof

Abstract

The present invention provides a tool steel and a preparation method thereof, aiming to solve the problems of high gas content, many inclusions and insufficient purity of existing hot working die steel. The chemical composition of the tool steel includes C: 0.34-0.38%, Mn: 0.80-1.50%, Si: 1.50-1.80%, Ni: 1.80-2.50%, Cr: 1.20-2.00%, Mo: 1.00-1.30%, V: 0.15-0.30%, and the content of harmful elements such as S, P, H, O, and N is strictly controlled. This type of steel has good toughness and plasticity while ensuring high strength, and its mechanical properties and wear resistance are significantly improved; the preparation method effectively improves the hot working performance and service life of the steel by accurately controlling the smelting process, optimizing the chemical composition, and reducing the gas content. The tool steel is particularly suitable for molds under high temperature and high pressure conditions, and has excellent mechanical properties and a long service life.

Description

Technical Field

[0001] The present invention relates to the technical field of alloy preparation, and in particular to a tool steel and a preparation method thereof. Background Art

[0002] Tool steel is widely used in the manufacture of various molds, and its performance requirements vary according to different working conditions. According to different uses, mold steel can be divided into three categories: cold work mold steel, hot work mold steel and plastic molding mold steel. Cold work mold steel is mainly used to manufacture molds for pressing and forming in a cold state; hot work mold steel is used to manufacture molds for pressure processing under high temperature; plastic mold steel has diverse performance requirements according to different types of plastics.

[0003] Cold working die steel is usually used to make dies that withstand cold working conditions, such as stamping dies, drawing dies, etc. Common cold working die steels include carbon tool steel, alloy tool steel, high-speed tool steel, etc. The key characteristics of this type of steel are good wear resistance and toughness, and the ability to withstand high pressure and impact.

[0004] Hot working die steel is widely used to manufacture dies that are processed in high temperature environments, such as hot forging dies, hot extrusion dies and die casting dies, etc. This type of steel usually has high heat resistance and thermal cracking resistance, can maintain strength and toughness at high temperatures, and meet the processing requirements of dies at high temperatures.

[0005] Hot working die steel is made by adding chemical elements such as C, Ni, Cr, Si, Nb, V, and Mo to steel. The chemical elements are reasonably matched and quenched to form a large amount of quenched martensite structure. After tempering, stable tempered martensite is formed. In addition, the carbides play a pinning role in the martensite dislocation structure, which greatly improves the strength, hardness, and plasticity.

[0006] According to the above design criteria and organizational requirements, a variety of hot working die steels have been developed. Table 1 shows a hot working die steel H13 (4Cr5MoSiV1), standard GB / T 1299-2014, and its chemical element mass percentage is shown in Table 1: Table 1 Chemical composition (wt%) of hot working die steel H13 (4Cr5MoSiV1)

[0007]

[0008] The steel grade H13 shown in Table 1 is C-Cr-Mo-Si-V steel, which is widely used in the world and has been extensively studied by many scholars in various countries. The steel has high hardenability and thermal cracking resistance. It contains a high content of carbon and vanadium, good wear resistance, relatively weak toughness, good heat resistance, good strength and hardness at high temperatures, high wear resistance and toughness, excellent total mechanical properties and high tempering resistance stability. It is widely used in hot working dies, such as extrusion dies, extrusion rods, etc.

[0009] The manufacturing process is: electric furnace + refining and pouring electrode outside the furnace → electroslag → fast forging machine → annealing → polishing.

[0010] The steel has the following problems: the steel is smelted through the electric furnace + external refining + electroslag smelting process, and the gas elements oxygen, nitrogen and hydrogen are relatively high, resulting in more oxides in the finished forgings. In addition, due to the high hydrogen content, the risk of white spots increases, resulting in reduced strength and toughness of the material, and even scrapping. Since the steel contains Nb and C elements, the steel ingots produced by electric furnace + external refining are prone to form CNNb inclusions, forming hard spots, poor plasticity, and reduced yield.

[0011] In summary, the die steel in the prior art, especially the hot working die steel, often has the following defects:

[0012] High gas element content: For example, high content of gas elements such as hydrogen, oxygen and nitrogen leads to more gas inclusions in the steel. Especially when the hydrogen content is high, it is easy to cause white spot defects, which will affect the strength and toughness of the steel and may even cause the steel to be scrapped. Existing smelting processes, especially the combination of electric furnace and refining outside the furnace with electroslag smelting, are difficult to effectively remove these gas elements.

[0013] Formation of hard inclusions: Certain elements (such as titanium and aluminum) may form hard inclusions such as TiN and AlN in steel, which have adverse effects on the plasticity and toughness of steel. Especially when elements such as carbon, vanadium and niobium exist in the steel at the same time, CNNb inclusions are easily formed, which further aggravates the brittleness of the material and reduces its bearing capacity.

[0014] Insufficient purity: Existing hot working die steel, especially when using traditional smelting processes (such as electric furnace smelting), has low purity. The residual elements and inclusions in the steel not only affect the mechanical properties of the steel, but also increase the instability during the processing, causing cracks, deformation and other problems in the mold during use.

[0015] Poor heat treatment effect: Although the mechanical properties of mold steel in the existing technology can be improved to a certain extent through tempering treatment, the toughness and wear resistance of the steel are still subject to certain limitations due to the presence of more impurities and inclusions in the steel.

[0016] Therefore, it has become a top priority to develop a new type of hot working die steel that can reduce gas content, reduce inclusions, improve steel purity, and optimize its mechanical properties and heat treatment effects. This can not only solve many problems existing in existing die steels, but also greatly improve the service life and processing efficiency of the die. Summary of the invention

[0017] The technical problem to be solved by the present invention is to provide a tool steel to solve the problems of insufficient strength and toughness, low purity and limited high temperature performance existing in conventional steel grades in the prior art.

[0018] In order to overcome the above defects of the prior art, the present invention provides a tool steel, the chemical composition of the tool steel includes the following by mass percentage:

[0019] C: 0.34~0.38%; Mn: 0.80~1.50%; Si: 1.50~1.80%; Ni: 1.80~2.50%; Cr: 1.20~2.00%; Mo: 1.00~1.30%; V: 0.15~0.30%; S≤0.001%; P≤0.003%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00010%; O: ≤0.0015%; N: ≤0.0010%; the balance is Fe and other unavoidable impurities,

[0020] Compared with the prior art, the tool steel of the present invention has the following advantages: through the chemical composition design of the present invention, the ratio and synergy of key elements are optimized, and the impurity and gas content are strictly controlled, which not only improves the strength and toughness matching and high temperature stability of the steel, but also significantly improves the thermal cracking resistance, fatigue resistance and service life, and successfully solves the performance defects of hot working die steel in the prior art, specifically including the following progress:

[0021] Optimized chemical composition design to improve steel performance:

[0022] High purity: The tool steel of the present invention significantly reduces the inclusion content by strictly controlling impurity elements such as S, P, Al, and Ti (S≤0.001%, P≤0.003%, Al≤0.015%, and Ti≤0.015%), reduces the adverse effects of E-type inclusions on strength and toughness, and improves the overall toughness and uniformity of the material;

[0023] Balance of strength and toughness: Further through the reasonable ratio of C, Mn and Si (C: 0.34-0.38%, Mn: 0.80-1.50%, Si: 1.50-1.80%), not only the strength is improved, but also the excellent toughness and fatigue resistance are maintained at high temperature, which is particularly suitable for the high strength requirements of hot working molds;

[0024] Furthermore, the synergistic effect of alloying elements improves hot cracking resistance and high temperature stability:

[0025] Composite strengthening effect of Cr, Mo and V: The combination of Cr (1.20-2.00%) and Mo (1.00-1.30%) improves the hardenability of the material, and improves the wear resistance and high temperature softening resistance; V (0.15-0.30%) forms dispersed carbides, effectively inhibiting grain growth, thereby further enhancing high temperature toughness and thermal cracking resistance;

[0026] Ni strengthening and toughening effect: The addition of Ni (1.80-2.50%) significantly improves the matrix toughness, optimizes the low-temperature impact properties, and provides stable mechanical properties under high temperature conditions;

[0027] Strict control of gas content, reducing defects and increasing service life: The gas impurity content is strictly limited to H≤0.00010%, O≤0.0015%, N≤0.0010%, which greatly reduces the white spots in the material and crack defects caused by other gas inclusions, and increases the service life of the material under high stress and high temperature conditions;

[0028] Excellent comprehensive performance to meet the needs of harsh working conditions: Under the strictly controlled chemical composition design, the steel type of the present invention shows an excellent strength-toughness matching relationship in a high-temperature environment, especially the significant improvement in impact toughness and fatigue resistance, making it widely used in high-temperature working environments such as extrusion dies, die-casting dies, and hot forging dies.

[0029] Another technical problem to be solved by the present invention is to provide a preparation method for the above-mentioned tool steel to solve the problems of insufficient purity of molten steel, high inclusion content, difficulty in effectively removing gas elements, and uneven strength and toughness of the finished product existing in the conventional preparation methods in the prior art.

[0030] In order to overcome the above defects of the prior art, the present invention provides a method for preparing tool steel, comprising the following steps:

[0031] S1: Smelting: The raw materials are batched according to the chemical composition ratio of tool steel, and the raw materials are smelted in a vacuum induction furnace. After refining, molten steel is obtained, and the electrode is obtained after the molten steel is poured;

[0032] S2: Vacuum consumable smelting: vacuum consumable smelting is performed on the electrode obtained in step S11 to obtain a steel ingot;

[0033] S3: Forging: The steel ingot obtained in step S2 is charged into a furnace for heat treatment and forging, wherein the starting forging temperature is 950-1100° C. and the final forging temperature is 800-900° C., to obtain a forged steel ingot;

[0034] S4: Heat treatment: The steel ingot forged in step S3 is subjected to quenching, primary tempering and secondary tempering in sequence to obtain tool steel, wherein the quenching temperature is 890-910°C, the primary tempering temperature is 500-550°C, and the secondary tempering temperature is 500-550°C.

[0035] Compared with the prior art, the method for preparing tool steel in the present invention has the following advantages:

[0036] Significantly improve the purity of steel: The preparation method of the present invention reduces the gas (H≤0.00010%, O≤0.0015%, N≤0.0010%) and inclusion content, reduces inclusion segregation, eliminates the tendency of white spots, and thus improves the purity and uniformity of steel through a dual process of vacuum induction melting combined with vacuum consumable smelting;

[0037] Optimized heat treatment process to enhance comprehensive performance: The forging and heat treatment process in the preparation method ensures the refinement of steel grains and uniform martensite structure through precise temperature control and holding time formula, which significantly improves the strength, toughness and fatigue resistance of steel;

[0038] Reducing crack and thermal crack tendency: The preparation method of the present invention reduces thermal stress accumulation and grain boundary segregation through strict temperature control and slow heating process during forging and heat treatment, and effectively reduces crack sensitivity;

[0039] Efficient process flow improves product performance consistency: The preparation method of the present invention, through dual smelting combined with optimized forging and heat treatment processes, ensures that the longitudinal and transverse mechanical properties of the tool steel tend to be consistent, and meets the comprehensive requirements for strength, toughness and high temperature resistance under harsh working conditions; through the technical improvement of the above-mentioned preparation method, the preparation method of the present invention significantly improves the purity, strength-toughness matching relationship and high-temperature stability of the tool steel, solves the key preparation problems in the prior art, and makes the tool steel of the present invention have broad application prospects.

[0040] In a possible implementation, in step S1, the temperature of the smelting raw material is 1520-1600°C, and the temperature of the molten steel pouring is 1540-1580°C.

[0041] Compared with the prior art, the above technical solution ensures that the molten steel can be completely melted by controlling the melting temperature at 1520-1600°C, while avoiding the risk of excessive volatilization and crucible material contamination caused by excessively high temperatures. This temperature range is conducive to the full implementation of gas removal processes such as deoxidation and denitrification, and makes the alloy elements dissolve evenly, improves the chemical composition uniformity of the molten steel, reduces the residual gas elements (such as O, N, H), and avoids structural defects caused by gas inclusions; the uniform molten state prevents segregation in the molten steel, forming a pure smelting basis; by controlling the temperature of molten steel pouring at 1540-1580°C, it can effectively ensure that the molten steel has appropriate fluidity and reduce eddy currents or slag rolls during the pouring process. At the same time, this temperature avoids the situation where the molten steel solidifies prematurely due to too low a temperature and the inclusions are redispersed due to too high a temperature, thereby improving the density of the cast finished electrode, significantly reducing casting defects such as pores and cracks, and providing high-quality initial electrodes for subsequent vacuum consumable smelting.

[0042] In a possible implementation, in step S1, the refining temperature is 1500-1570° C., and during the refining, the vacuum degree in the vacuum induction furnace is ≤5 Pa, and the refining period is maintained for ≥60 min.

[0043] Compared with the prior art, the above technical scheme is adopted to control the refining temperature to 1500-1570°C, which can place the molten steel in the optimal physical and chemical reaction activity range, effectively promote the removal of nitrogen, oxygen and hydrogen in the molten steel, and inhibit the dissolution contamination of elements on the inner wall of the crucible, and the temperature range is conducive to the full dissolution and uniform diffusion of alloy elements, while avoiding the volatilization of elements caused by high temperature or incomplete refining caused by low temperature; further, by keeping the vacuum degree in the vacuum induction furnace ≤5Pa and the refining period ≥60min, the vacuum environment effectively improves the gas content problem of the molten steel, reduces the risk of defects such as hydrogen-induced cracks and white spots, and by fully extending the refining reaction time, improves the deoxidation and denitrification efficiency, and avoids the influence of residual inclusions on the quality of steel.

[0044] In a possible implementation, in step S2, during the vacuum consumable smelting process, the vacuum degree is ≤0.5Pa, and the conditions of the vacuum consumable smelting are: the current in the initial stage is 4.0-11.5KA, the melting rate in the melting stage is 2.5-6.5kg / min, and the current in the feeding stage is 2.5-6.5KA.

[0045] Compared with the prior art, the above technical scheme is adopted to control the vacuum degree ≤0.5Pa, which significantly reduces the partial pressure of gas elements (such as hydrogen, oxygen, and nitrogen) during the smelting process, helps the gas to fully escape from the molten steel, and reduces the gas content in the ingot; and further, by controlling the current range in the arc starting stage, it is possible to avoid excessive fluctuations in the molten pool temperature, ensure the stability of the melting process and reduce segregation in the early stage of melting. A moderate melting speed is adopted in the melting stage, which can avoid component segregation caused by excessively fast melting speed, and prevent low process efficiency and unnecessary energy consumption caused by too slow melting speed. In the shrinkage feeding stage, the solidification rate of the molten steel is controlled by reducing the current, the segregation trend in the liquid steel is reduced, and the density and quality of the ingot head are ensured.

[0046] In a possible implementation, in step S3, the conditions for the furnace heat treatment are as follows: controlling the charging temperature of the steel ingot to be ≤600°C, heating it to 800-1000°C for the first time, and the rate of the first heating is ≤200°C / h, and the first insulation is 1-4h; then heating it for the second time, and the heating rate of the second heating is ≤200°C / h, and when the second heating reaches the steel ingot temperature of 1100-1180°C, insulation is carried out for 1-7h.

[0047] Compared with the prior art, the above technical solution can prevent the accumulation of thermal stress caused by uneven heating of the ingot in the initial stage, and slow down the embrittlement of the grain boundaries. The slow heating rate in the first heating avoids the stress concentration caused by the excessive temperature gradient of the ingot, and the long-term heat preservation in this temperature range helps to homogenize the microstructure of the ingot and reduce the residual stress; in the second heating, it is ensured that the ingot is completely austenitized, and the temperature difference between the inside and outside of the ingot is too large by slow heating, reducing the risk of grain coarsening; the above two heating and heat preservation of the heat treatment work together, and the microstructure inside the ingot is homogenized through the design of staged heating and heat preservation, which reduces the grain growth and segregation phenomenon, provides a guarantee for the stability of the performance of the final product, solves the problems of excessive internal stress, grain coarsening and poor uniformity of the ingot caused by improper control of the heat treatment process in the prior art, and lays a foundation for the subsequent forging and improvement of the quality of the finished product.

[0048] In a possible implementation, in step S4,

[0049] The quenching treatment time is the holding time t=(heating coefficient n×R)+60, wherein R is the cross-sectional radius of the steel ingot obtained by the forging, in mm, the heating coefficient n=2.2-2.8, and the holding time t is in min;

[0050] The holding time of the primary tempering is t=(heating coefficient η×R)+120, wherein R is the cross-sectional radius of the steel ingot obtained by the forging, in mm, the heating coefficient n=2.2-2.8, and the holding time t is in min;

[0051] The holding time after the secondary treatment is t=(heating coefficient n×R)+120, wherein R is the cross-sectional radius of the steel obtained by forging, in units of mm, the heating coefficient n=2.2-2.8, and the holding time t is in units of min.

[0052] Compared with the prior art, the above technical solution is adopted to relate the holding time to the cross-sectional radius of the steel ingot through a specific formula in the present invention, and the above formula is also related to the preparation method of the present invention, so that the heat treatment parameters can be dynamically adapted to steel ingots of different specifications. This design reduces the problem of insufficient heating or overheating of large or small steel ingots caused by a single time parameter; the precise holding time in the quenching stage can ensure complete austenitization and reduce the residual stress in the martensitic transformation; the appropriate holding time in the tempering stage can fully temper the martensite, improve the organizational stability, and make the steel strong. The strength, toughness and wear resistance are optimized, and the internal stress is effectively released, reducing the risk of deformation and cracking after heat treatment. In terms of the control of the holding time, the tempering stage promotes the formation of fine precipitates through a longer holding time (longer than the quenching holding time), and repairs the micro cracks that may be left in the quenching stage, further significantly improving the comprehensive mechanical properties of the steel, especially the impact toughness and fatigue performance, solving the technical problem in the prior art that the heat treatment time is difficult to match the ingot specifications, and significantly improving the mechanical properties of the steel and the adaptability and reliability of the preparation process.

[0053] In a possible implementation, in step S4, the cooling method of the quenching is oil cooling, the cooling method of the primary tempering is air cooling, and the cooling method of the secondary tempering is air cooling.

[0054] Compared with the prior art, the above technical scheme is adopted to quickly obtain martensitic structure through oil cooling in the quenching stage to ensure strength performance, and to achieve uniform cooling, stabilize the structure, and release stress through air cooling in the tempering stage. The synergistic effect of the multi-stage cooling process enables the steel to achieve the best balance between strength and toughness. It not only solves the problems of cracks and insufficient hardness caused by too fast or too slow quenching cooling speed in the prior art, but also further optimizes the comprehensive performance and application reliability of tool steel by precisely controlling the cooling process. DETAILED DESCRIPTION

[0055] First, those skilled in the art should understand that these implementations are only used to explain the technical principles of the embodiments of the present application, and are not intended to limit the protection scope of the embodiments of the present application. Those skilled in the art can make adjustments to them as needed to adapt to specific application scenarios.

[0056] The present invention provides a tool steel, wherein the chemical composition of the tool steel comprises, by mass percentage:

[0057] C: 0.34~0.38%; Mn: 0.80~1.50%; Si: 1.50~1.80%; Ni: 1.80~2.50%; Cr: 1.20~2.00%; Mo: 1.00~1.30%; V: 0.15~0.30%; S≤0.001%; ​​P≤0.003%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00010%; O: ≤0.0015%; N: ≤0.0010%; the balance is Fe and other inevitable impurities.

[0058] The design principle of the steel grade described in the present invention is as follows:

[0059] As a new type of hot working die steel, extremely low residual elements and the interaction between alloy elements and the matching of addition amounts are very important.

[0060] The role of nickel in Benxi steel is reflected in the following aspects: Ni is an element that expands the austenite phase region. During the cooling process after austenitization, it strongly inhibits the transformation of austenite to pearlite and bainite, and improves the hardenability of martensite; Ni reduces the temperature of the transformation of supercooled austenite to martensite, increases the content of residual austenite, and is beneficial to toughness; Ni can increase the stacking fault energy of the steel matrix, making it easier for screw dislocations to produce cross slip, thereby improving toughness.

[0061] The role of manganese in Benxi steel is reflected in the following aspects: Mn, like Ni, is an element that expands the austenite phase region. Mn reduces the temperature at which supercooled austenite transforms to martensite, increases the content of retained austenite, and is beneficial to toughness.

[0062] The role of vanadium in this steel is reflected in the following aspects: V is a strong carbide-forming element, forming a stable VC phase, organizing the growth of austenite grains during quenching and heating, thereby refining the martensite structure, improving the toughness of tempered martensite while increasing the strength, and increasing the cleavage fracture stress.

[0063] The role of chromium and molybdenum in this steel is reflected in the following aspects: Cr and Mo delay the transformation of supercooled austenite to pearlite during quenching and cooling, thus improving the hardenability of steel; during tempering, they enter carbides to form alloy carbides, thus increasing the stability of carbides and improving the tempering resistance of steel. At the same time, Cr can also improve the corrosion resistance of steel.

[0064] The role of aluminum and titanium in this steel is reflected in the following aspects: Al and Ti are easy to combine with N in the steel to form AlN and TiN, which increases the number of E-type non-metallic inclusions and leads to a decrease in the strength and toughness of the steel. In order to achieve the theoretical matrix strength of this steel and improve the purity of the steel, the content of Al and Ti must be strictly controlled as low as possible.

[0065] The role of phosphorus and sulfur in Ben Steel is reflected in the following aspects: phosphorus will reduce the cold bending performance of steel and deteriorate weldability. At the same time, phosphorus will improve the strength, hardness, wear resistance and corrosion resistance of steel, but these positive effects are usually offset by the cold brittleness caused by it. Phosphorus dissolves in ferrite. Due to its large atomic diameter, it will cause ferrite lattice distortion, thereby significantly reducing the plasticity and toughness of steel, especially in low temperature environment. This reduction is particularly obvious. In addition, phosphorus is easy to segregate during the crystallization process of steel and is difficult to eliminate by heat treatment, which increases the risk of cold brittleness of steel. Hazards of sulfur: Sulfur is easy to cause brittle cracking during the hot working of steel, a phenomenon called hot brittleness. The presence of sulfur will also reduce the impact toughness, fatigue strength, weldability and corrosion resistance of steel. Even if sulfur exists in trace amounts, it is harmful to steel, so the sulfur content needs to be strictly controlled. Sulfur will form brittle compounds in the weld, resulting in brittle welded joints and even cracking during welding. Sulfur also reduces the cold forming properties of steel and affects its plastic deformation capacity, making it easy for cracks and fractures to occur during bending, stamping and other processing processes. The presence of phosphorus and sulfur in steel will have a negative impact on its physical properties and processing properties, so its content in steel needs to be strictly controlled.

[0066] In summary, harmful elements P, S, Al, Ti and gases H, O, N, etc. are regarded as impurity elements and are kept as low as possible. The raw materials are intermediate alloys and metal materials of Ben Steel or similar steels. After vacuum induction, electroslag remelting and vacuum consumable remelting, high-purity medium alloy ultra-high strength steel is made. The preparation method is as follows:

[0067] The present invention also provides a method for preparing the tool steel, comprising the following steps:

[0068] S1: Smelting: The raw materials are proportioned according to the chemical composition ratio of the tool steel, and the raw materials are melted in a vacuum induction furnace. The raw material requirements for the vacuum induction furnace include: adding high-purity pure iron, pure iron requires S≤0.002%, P≤0.005% or refined steel smelted by electric furnace + LF+VOD+VD, and the residual elements S, P, and Al of refined steel are required to be ≤0.001%; ​​then add very high-purity metal molybdenum, metal nickel, metal chromium, metal niobium, metal manganese or electrolytic manganese. Check the surface of the raw materials before loading the furnace. Rust, oil stains, and oxidation are not allowed. If there is any, it must be cleaned. During the smelting process, the material is slowly cooled at low temperature during the melting period. Vacuum induction furnace smelting can improve the degassing effect by increasing the steel liquid level. Therefore, when slow and low-power chemistry is used, a good degassing effect can be achieved. During the refining process, after the refining period, when the gas meets the requirements, that is, when O≤15ppm, N≤15ppm, and H≤1ppm, add Ar≥8000Pa and start adding CrN. After stirring for 5-10 minutes, add metal Mn or electrolytic Mn. Continue refining. After the end, molten steel is obtained. After the molten steel is poured, the electrode is obtained. The electrode requires slow cooling (including sand cooling, hood cooling, pit cooling, furnace slow cooling and other cooling methods) and then vacuum consumable smelting after polishing and flattening. It can effectively reduce the content of chemical element S in steel, and improve non-metallic inclusions to ensure the purity of steel, and prepare for improving the strength and toughness of steel. ;

[0069] S2: Vacuum consumable smelting: The electrode obtained in step S11 is subjected to vacuum consumable smelting to obtain a steel ingot. During the vacuum consumable smelting process, the vacuum consumable furnace requires helium cooling to accelerate cooling, and condensation while melting. The use of droplet pulse technology is beneficial to improving chemical composition segregation and ingot shape segregation, and improving the purity of the steel ingot. After the vacuum consumable furnace is remelted into an ingot, slow cooling (including hood cooling, sand cooling, pit cooling, and furnace cooling) is required for ≥24h to polish or turn the surface of the steel ingot;

[0070] S3: Forging: The steel ingot obtained in step S2 is charged into a furnace for heat treatment and forging, wherein the starting forging temperature is 950-1100° C. and the final forging temperature is 800-900° C. Within this temperature range, the steel can be formed in the best thermoplastic zone, thereby avoiding forging cracking and obtaining a forged steel ingot;

[0071] S4: Heat treatment: The steel ingot forged in step S3 is subjected to quenching, primary tempering and secondary tempering in sequence to obtain tool steel, wherein the quenching temperature is 890-910°C, the primary tempering temperature is 500-550°C, and the secondary tempering temperature is 500-550°C.

[0072] As a preferred solution, in step S1, the temperature of the smelting raw material is 1520-1600°C, and the temperature of the molten steel pouring is 1540-1580°C. If the pouring temperature is too low, the molten steel solidifies and the pouring cannot be completed, and if the pouring temperature is too high, the electrode is prone to cracking.

[0073] As a preferred solution, in step S1, the refining temperature is 1500-1570°C. If the refining temperature is too low, no refining effect will be achieved. If the temperature is too high, Al and Ti in the crucible will be reduced to the molten steel. During the refining period, the vacuum degree in the vacuum induction furnace is ≤5Pa, and the refining period is maintained for ≥60min.

[0074] As a preferred solution, in step S2, during the vacuum consumable smelting process, the vacuum degree is ≤0.5Pa, thereby achieving remelting. The degassing vacuum consumable furnace smelting specifies different melting rates according to different ingot types. The melting period is based on the melting rate. The melting rate is as low as possible to improve the segregation of chemical elements. The conditions for vacuum consumable smelting are: the current in the initial stage is 4.0~11.5KA, the melting rate in the melting stage is 2.5~6.5kg / min, and the current in the feeding stage is 2.5~6.5KA, which can effectively prevent the occurrence of segregation.

[0075] As a preferred scheme, in step S3, the conditions for the furnace heat treatment are: controlling the charging temperature of the steel ingot to be ≤600°C, heating it to 800-1000°C for the first time, and the rate of the first heating is ≤200°C / h, and the first insulation is 1-4h; then heating it for the second time, the heating rate of the second heating is ≤200°C / h, and when the second heating reaches the steel ingot temperature of 1100-1180°C, insulation is carried out for 1-7h.

[0076] As a preferred solution, in step S4,

[0077] The quenching treatment time is the holding time t=(heating coefficient n×R)+60, wherein R is the cross-sectional radius of the steel ingot obtained by the forging, in mm, the heating coefficient n=2.2-2.8, and the holding time t is in min;

[0078] The holding time of the primary tempering is t=(heating coefficient η×R)+120, wherein R is the cross-sectional radius of the steel ingot obtained by the forging, in mm, the heating coefficient n=2.2-2.8, and the holding time t is in min;

[0079] The holding time after the secondary treatment is t=(heating coefficient n×R)+120, wherein R is the cross-sectional radius of the steel obtained by forging, in units of mm, the heating coefficient n=2.2-2.8, and the holding time t is in units of min.

[0080] As a preferred solution, in step S4, the cooling method of the quenching is oil cooling, the cooling method of the primary tempering is air cooling, and the cooling method of the secondary tempering is air cooling.

[0081] In combination with the above-mentioned implementation modes, embodiments combined with specific data are provided below to describe the above-mentioned technical solutions of the present invention in detail:

[0082] Embodiment 1:

[0083] This embodiment provides a tool steel and a preparation method thereof, wherein the tool steel includes the following components in percentage by mass:

[0084] C: 0.36%, Mn: 1.15%, Si: 1.65%, Ni: 2.15%, Cr: 1.60%, Mo: 1.15%, V: 0.23%, S: 0.0008%, P: 0.002%, Al: 0.010%, Ti: 0.010%, H: 0.00008%, O: 0.0010%, N: 0.0008%, and the balance is Fe and other inevitable impurities.

[0085] The preparation method comprises the following steps:

[0086] S1: Smelting

[0087] The raw materials are prepared according to the above chemical composition ratio, and high-purity pure iron and high-purity metal molybdenum, nickel, chromium, niobium and electrolytic manganese are added. The raw materials are smelted in a vacuum induction furnace, the melting temperature is 1560°C, the refining temperature is 1530°C, the vacuum degree is controlled at 4Pa, the refining time is 65min, 8000Pa of argon is filled after refining, CrN is added and stirred for 5min, and then electrolytic manganese is added, the steel liquid pouring temperature is controlled at 1560°C, and the electrode is obtained after pouring.

[0088] S2: Vacuum consumable smelting

[0089] The above-mentioned electrode was subjected to vacuum consumable smelting, the vacuum degree was controlled to be 0.5Pa, the current was 8KA in the initial stage, the melting speed was 5kg / min in the melting stage, and the current was 5KA in the feeding stage to obtain a steel ingot.

[0090] S3: Forging

[0091] The steel ingot is charged into a furnace for heating, the charging temperature is controlled to be ≤600°C, the temperature is raised to 900°C at a heating rate of 150°C / h, and the temperature is kept for 2h; the temperature is further raised to 1150°C at a rate of 180°C / h, and the temperature is kept for 4h. The starting forging temperature is controlled to be 1000°C, the final forging temperature is 850°C, and the steel ingot is slowly cooled to room temperature after forging to obtain the forged steel ingot.

[0092] S4: Heat treatment

[0093] The forged steel ingot is subjected to quenching, primary tempering and secondary tempering in sequence:

[0094] The quenching temperature is 900℃, the holding time is t=2.5×R+60min, and the cooling method is oil cooling;

[0095] The primary tempering temperature is 520℃, the holding time is t=2.5×R+120min, and the cooling method is air cooling;

[0096] The secondary tempering temperature is 520°C, the holding time is t=2.5×R+120min, and the cooling method is air cooling.

[0097] Embodiment 2:

[0098] This embodiment provides a tool steel and a preparation method thereof, wherein the tool steel includes the following components in percentage by mass:

[0099] C: 0.34%, Mn: 0.80%, Si: 1.50%, Ni: 1.80%, Cr: 1.20%, Mo: 1.00%, V: 0.15%, S: 0.0010%, P: 0.003%, Al: 0.015%, Ti: 0.015%, H: 0.00010%, O: 0.0015%, N: 0.0010%, and the balance is Fe and other inevitable impurities.

[0100] The preparation method comprises the following steps:

[0101] S1: Smelting

[0102] The raw materials are prepared according to the above chemical composition ratio, and high-purity pure iron and high-purity metal molybdenum, nickel, chromium, niobium and electrolytic manganese are added. The raw materials are smelted in a vacuum induction furnace, the melting temperature is 1520°C, the refining temperature is 1500°C, the vacuum degree is controlled at 5Pa, the refining time is 60min, 8000Pa of argon is filled after refining, CrN is added and stirred for 5min, and then electrolytic manganese is added, the steel liquid pouring temperature is controlled at 1540°C, and the electrode is obtained after pouring.

[0103] S2: Vacuum consumable smelting

[0104] The above-mentioned electrode was subjected to vacuum consumable smelting, the vacuum degree was controlled at 0.5Pa, the current was 4.0KA in the initial stage, the melting speed was 2.5kg / min in the melting stage, and the current was 2.5KA in the feeding stage to obtain a steel ingot.

[0105] S3: Forging

[0106] The steel ingot is charged into a furnace for heating, the charging temperature is controlled to be ≤600°C, the temperature is raised to 800°C at a heating rate of 200°C / h, and the temperature is kept for 4 hours; the temperature is further raised to 1100°C at a rate of 200°C / h, and the temperature is kept for 1 hour. The start forging temperature is controlled to be 950°C, the final forging temperature is 800°C, and the steel ingot is slowly cooled to room temperature after forging to obtain a forged steel ingot.

[0107] S4: Heat treatment

[0108] The forged steel ingot is subjected to quenching, primary tempering and secondary tempering in sequence:

[0109] The quenching temperature is 890℃, the holding time is t=2.2×R+60min, and the cooling method is oil cooling;

[0110] The primary tempering temperature is 500°C, the holding time is t=2.2×R+120min, and the cooling method is air cooling;

[0111] The secondary tempering temperature is 500°C, the holding time is t=2.2×R+120min, and the cooling method is air cooling.

[0112] Embodiment 3:

[0113] This embodiment provides a tool steel and a preparation method thereof, wherein the tool steel includes the following components in percentage by mass:

[0114] C: 0.38%, Mn: 1.50%, Si: 1.80%, Ni: 2.50%, Cr: 2.00%, Mo: 1.30%, V: 0.30%, S: 0.0005%, P: 0.002%, Al: 0.005%, Ti: 0.005%, H: 0.00005%, O: 0.0008%, N: 0.0005%, and the balance is Fe and other inevitable impurities.

[0115] The preparation method comprises the following steps:

[0116] S1: Smelting

[0117] The raw materials are prepared according to the above chemical composition ratio, and high-purity pure iron and high-purity metal molybdenum, nickel, chromium, niobium and electrolytic manganese are added. The raw materials are smelted in a vacuum induction furnace, the melting temperature is 1600°C, the refining temperature is 1570°C, the vacuum degree is controlled at 4Pa, the refining time is 70min, 8000Pa of argon is filled after refining, CrN is added and stirred for 10min, and then electrolytic manganese is added, the steel liquid pouring temperature is controlled at 1580°C, and the electrode is obtained after pouring.

[0118] S2: Vacuum consumable smelting

[0119] The above-mentioned electrode was subjected to vacuum consumable smelting, the vacuum degree was controlled at 0.5Pa, the current was 11.5KA in the initial stage, the melting speed was 6.5kg / min in the melting stage, and the current was 6.5KA in the feeding stage to obtain a steel ingot.

[0120] S3: Forging

[0121] The steel ingot is charged into a furnace for heating, the charging temperature is controlled to be ≤600°C, the temperature is raised to 1000°C at a heating rate of 180°C / h, and the temperature is kept for 2 hours; the temperature is further raised to 1180°C at a rate of 200°C / h, and the temperature is kept for 7 hours. The start forging temperature is controlled to be 1100°C, the final forging temperature is 900°C, and the steel ingot is slowly cooled to room temperature after forging to obtain a forged steel ingot.

[0122] S4: Heat treatment

[0123] The forged steel ingot is subjected to quenching, primary tempering and secondary tempering in sequence:

[0124] The quenching temperature is 910℃, the holding time is t=2.8×R+60min, and the cooling method is oil cooling;

[0125] The primary tempering temperature is 550°C, the holding time is t=2.8×R+120min, and the cooling method is air cooling;

[0126] The secondary tempering temperature is 550°C, the holding time is t=2.8×R+120min, and the cooling method is air cooling.

[0127] In Table 1 and Table 2, H12 is the 4Cr5MoSiV1 steel grade in the background technology.

[0128] Low-power test results: The finished steel material of the present invention was sampled twice and subjected to room temperature tensile test according to GB / T228. The results are shown in Appendix 1, which shows the mechanical properties of tool steel, including tensile strength, yield strength, elongation, area reduction and impact toughness (impact U). The results compare the performance of different embodiments (1-3) with H13 steel. Embodiments 1, 2, and 3 are significantly better than H13 steel in tensile strength, yield strength, elongation and area reduction; in tensile strength: the embodiment reaches a maximum of 2030MPa, while H13 steel is only 1760MPa. In terms of yield strength, the maximum yield strength of the embodiment is 1800MPa, while that of H13 steel is 1435MPa; in terms of elongation and area reduction: the embodiment shows better plasticity and toughness, with a maximum elongation of 16% and a maximum area reduction of 38%, which is significantly better than H13 steel (elongation 9%, area reduction 28%); in terms of impact toughness: the maximum impact toughness of the embodiment is 60J, while that of H13 steel is only 19J. Table 1 shows that the tool steel provided by the present invention has significant improvements in strength, plasticity and toughness, and the longitudinal and transverse differences in mechanical properties are small, showing higher comprehensive performance.

[0129] Table 1 Macroscopic test results of Examples 1-3 and H13 alloy

[0130]

[0131] Metallographic inspection results test: The finished steel material of the present invention was sampled and inspected for non-metallic inclusions according to GB / T10561. The results are shown in Appendix 2. Table 2 shows the content of non-metallic inclusions, and the evaluation is carried out according to the fine and coarse grades of four types of non-metallic inclusions A, B, C, and D: In Example 1 and Example 2, the grades of various non-metallic inclusions are 0 or close to 0, and there are only a very small amount of D-type inclusions (0.5 grade), which indicates that the purity of the steel is very high, while in H13 steel, the grades of non-metallic inclusions of types A, B, and D are significantly higher than those of the embodiments, especially A (1.0 grade) and D (1.5 grade), indicating that the purity of H13 steel is relatively low. Table 2 shows that the tool steel of the present invention effectively reduces the content of non-metallic inclusions and improves the purity of the steel through processes such as refining and vacuum consumable smelting, thereby helping to improve the mechanical properties and service life of the steel.

[0132] Table 2: Non-metallic inclusions of Examples 1-3 and H13 alloy

[0133]

[0134] In summary, the present invention provides a process for smelting and manufacturing tool steel using vacuum induction + vacuum consumable remelting process to produce the new hot working die steel described in the present invention. The die steel produced by this method, combined with the special mass ratio of the tool steel in the present invention, has a low content of impurity elements in the prepared steel, good steel quality, high strength and good toughness, and solves the problems existing in the prior art.

[0135] In the description of the present application, the description with reference to the terms "one embodiment", "some embodiments", "in the present embodiment", "specific example", or "some examples" etc. means that the specific features, mechanisms, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, mechanisms, materials or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0136] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be based on the protection scope of the claims.

Claims

1. A tool steel, characterized in that: The chemical composition of the tool steel includes by mass percentage: C: 0.34~0.38%; Mn: 0.80~1.50%; Si: 1.50~1.80%; Ni: 1.80~2.50%; Cr: 1.20~2.00%; Mo: 1.00~1.30%; V: 0.15~0.30%; S≤0.001%; ​​P≤0.003%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00010%; O: ≤0.0015%; N: ≤0.0010%; the balance is Fe and other inevitable impurities.

2. A method for preparing the tool steel according to claim 1, characterized in that: The following steps are involved: S1: Smelting: The raw materials are batched according to the chemical composition ratio of tool steel, and the raw materials are smelted in a vacuum induction furnace. After refining, molten steel is obtained, and the electrode is obtained after the molten steel is poured; S2: Vacuum consumable smelting: vacuum consumable smelting is performed on the electrode obtained in step S11 to obtain a steel ingot; S3: Forging: The steel ingot obtained in step S2 is charged into a furnace for heat treatment and forging, wherein the starting forging temperature is 950-1100° C. and the final forging temperature is 800-900° C., to obtain a forged steel ingot; S4: Heat treatment: The steel ingot forged in step S3 is subjected to quenching, primary tempering and secondary tempering in sequence to obtain tool steel, wherein the quenching temperature is 890-910°C, the primary tempering temperature is 500-550°C, and the secondary tempering temperature is 500-550°C.

3. The method for preparing tool steel according to claim 2, characterized in that: In the step S1, the temperature of the smelting raw material is 1520-1600°C, and the temperature of the molten steel pouring is 1540-1580°C.

4. The method for preparing tool steel according to claim 2, characterized in that: In the step S1, the refining temperature is 1500-1570°C, and during the refining period, the vacuum degree in the vacuum induction furnace is ≤5Pa, and the refining period is maintained for ≥60min.

5. The method for preparing tool steel according to claim 2, characterized in that: In the step S2, during the vacuum consumable smelting process, the vacuum degree is ≤0.5Pa, and the conditions of the vacuum consumable smelting are: the current in the initial stage is 4.0-11.5KA, the melting speed in the melting stage is 2.5-6.5kg / min, and the current in the feeding stage is 2.5-6.5KA.

6. The method for preparing tool steel according to claim 2, characterized in that: In the step S3, the conditions for the furnace heat treatment are: controlling the charging temperature of the steel ingot to be ≤600°C, heating it to 800-1000°C for the first time, and the speed of the first heating is ≤200°C / h, and the first insulation is 1-4h; then heating it for the second time, the heating speed of the second heating is ≤200°C / h, and when the temperature of the steel ingot reaches 1100-1180°C for the second time, insulation is carried out for 1-7h.

7. The method for preparing tool steel according to claim 2, characterized in that: In the step S4, The quenching treatment time is the holding time t=(heating coefficient n×R)+60, wherein R is the cross-sectional radius of the steel ingot obtained by the forging, in mm, the heating coefficient n=2.2-2.8, and the holding time t is in min; The holding time of the primary tempering is t=(heating coefficient η×R)+120, wherein R is the cross-sectional radius of the steel ingot obtained by the forging, in mm, the heating coefficient n=2.2-2.8, and the holding time t is in min; The holding time after the secondary treatment is t=(heating coefficient n×R)+120, wherein R is the cross-sectional radius of the steel obtained by forging, in units of mm, the heating coefficient n=2.2-2.8, and the holding time t is in units of min.

8. The method for preparing tool steel according to claim 7, characterized in that: In the step S4, the cooling method of the quenching is oil cooling, the cooling method of the primary tempering is air cooling, and the cooling method of the secondary tempering is air cooling.

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