In the fifties, China's steel used in molds was entirely based on foreign steel grades. In the 1960s, in order to save raw materials and improve the accuracy of blanks, there were rapid developments in the absence of cutting processes and precision forming technologies. In order to increase production efficiency, many high-efficiency press processing equipment was used , and forging hammers were gradually replaced by presses. The performance of the original die steel often cannot meet the high performance requirements of the service conditions, which affects the service life of the die and the popularization and application of the new equipment for pressure processing. At the end of the 1970s, the use of precision and large-scale engineering plastic products became increasingly widespread. The demand for steel for plastic molds increased sharply, and new requirements were also put forward for the performance of plastic mold steels. At that time, there was no steel for plastic molds in China.
Since the 1960s, with the support of the relevant ministries and commissions of the country, Chinese scientists and technicians have developed a number of new tool steels in combination with the national conditions. Some of the new steels with excellent performance and relatively good process performance have been subjected to the manufacture and use of molds. welcome. During this period, some foreign steel grades were also introduced. Some of the steel grades passed trial production and achieved good results. For some cold working die steels and hot die steels with better results, the relevant departments also organized performance comparison test studies and proposed suggestions for selection and application. In order to meet the needs of high wear-resisting and long-life molds, at the end of the 1950s, China's cemented carbides had rapid development. At the same time, a variety of steel-bonded hard alloys were developed and used as molds to achieve good results.
This article is divided into cold work die steel, hot work die steel, plastic die steel, hard alloy and steel cemented carbide, mold heat treatment, outlook and recommendations in six parts.
1 Cold Work Die Steel At present, cold work die steel commonly used in China is still the old steel number of low alloy tool steel CrWMn and high carbon and high chromium tool steel Cr12MoV and Cr12. CrWMn steel has appropriate hardenability and wear resistance, heat treatment deformation is small, but CrWMn steel after forging needs to strictly control the cooling rate, and the use of appropriate heat treatment, so that the carbides were uniformly fine granular, distributed on the substrate, Otherwise it is easy to form network carbides, leading to chipping and cracking of the mold in use. High-carbon high-chromium tool steel has high wear resistance, but its carbide segregation is more serious, resulting in reduced directionality and toughness of deformation. Through repeated dialing, the degree of segregation can be improved to some extent.
In 1981, China introduced the internationally used high-carbon high-chromium tool steel D2 (Cr12Mo1V1). Compared with Cr12MoV steel, the carbide segregation of D2 steel is slightly improved compared with Cr12MoV, the strength and toughness are slightly improved, and the service life of D2 steel mold is also improved to varying degrees [4]. High-speed steels (mainly W6Mo5Cr4V2 and W18Cr4V) have higher wear resistance and strength, and are often used to make molds, but their toughness cannot meet the needs of molds with large and large impact loads.
In order to improve the toughness of these steels, China has developed new cold die steels such as:
1.1 Low-alloy cold work die steel The main features of this type of steel are good processability, low quenching temperature, small heat treatment distortion, good toughness, and appropriate wear resistance. Such as GD (6CrMnNiMoVSi), 7CrSiMnMoV (abbreviated CH), DS steel. GD steel is used to make cold stamping die with easy chipping and fracture, which has a high service life. The composition of CH steel is the same as that of Japan's SX105V steel. It is a flame-hardened steel and is often used to make mold parts for production lines such as automobiles. When the flame is quenched, the cutting edge of the mold edge is heated and hardened layer has a high toughness. The substrate is cushioned so that the mold has a high service life. DS steel is an impact cold working tool steel, its impact toughness is significantly better than the commonly used high toughness tool steel 6CrW2Si blade.
1.2 Base steel Base steel generally refers to steel whose composition is the same as that of the base body in the quenched structure of high speed steel. The United States and Japan studied base steels Vasco MA, Vasco Matrix I, and MOD2 in the early 1970s, which are equivalent to the M2 and M36 high-speed steel substrates, but have not been widely used. China has developed a number of matrix steels, such as 65Cr4W3Mo2VNb (abbreviated 65Nb), 65W8Cr4VTi (abbreviated LM1) 65Cr5Mo3W2VSiTi (abbreviated LM2) steel. The main characteristic of these matrix steels is that their carbon content is slightly higher than the carbon content of the matrix to increase primary carbides and wear resistance. A small amount of strong carbides are also added to form elemental niobium or titanium to form relatively stable carbonization. The product prevents grain growth during quenching and improves the process performance of the steel. This kind of matrix steel has been widely used in the production of cold extrusion, thick plate cold punch, cold heading and other molds, especially for large complex molds used in difficult-to-deform materials. It can also be used as a black metal extrusion mold.
1.3 High-toughness wear-resistant cold work die steel In order to improve the carbide segregation of Cr12 cold work die steel, improve its toughness, and further increase the wear resistance of steel, China has done a lot of research work and developed a lot of new Steel grades such as LD[8], ER5[9], and GM[10] steels. In these steels, the chromium content is appropriately reduced to improve the segregation of carbides, increase the content of tungsten, molybdenum, and vanadium to increase the ability of secondary hardening and increase the wear resistance. Compared with Cr12 cold work die steel, the carbide segregation of this kind of steel is improved and has higher toughness. This kind of steel has better wear resistance than Cr12 type cold work die steel, so the manufactured die has a higher life span and is more suitable for the use of high-speed punch presses and multi-station punch presses.
2 hot work die steel commonly used in China hot work die steel 5CrMnMo, 5CrNiMo and 3Cr2W8V steel.
5CrNiMo steel is mainly used as large and medium-sized forging dies. However, its hardenability is not high enough, and its tempering stability is not high. Its performance cannot meet the performance requirements of the large section forging dies. 3Cr2W8V steel is widely used as a die-casting mold for black and nonferrous metal hot extrusion die and Cu, Al alloy. This kind of steel has high thermal stability and the use temperature reaches 650°C. However, the tungsten type hot work die steel has low thermal conductivity and poor cold and hot fatigue resistance.
In the early 1980s, China introduced the commonly used chromium hot work die steel H13 (4Cr5MoSiV1), H13 steel has good cold and hot fatigue resistance, when the use temperature does not exceed 600? C, instead of 3Cr2W8V steel, die life has a substantial As a result, H13 steel has been rapidly applied and its output has surpassed 3Cr2W8V steel.
In order to adapt to the new requirements of pressure processing and new equipment, the higher toughness and thermal stability requirements of die steel, China has developed a lot of new hot work die steels, mainly including:
2.1 hot forging die steel domestic in the eighties, the hardenability of 5CrNiMo steel can not meet the needs of the large cross-section forging die and the use of temperature does not exceed 500? C, the domestic and foreign steel has done a lot of analysis and research . The research shows that the contents of Cr, Ni and Mo in 5CrNiMoV are higher than that of domestic 5CrNiMo steel, and contain a small amount of V. Therefore, the hardenability and tempering stability of the 5CrNiMoV steel are higher than that of domestic 5CrNiMo steel, and 5CrNiMoV is recommended. Steel, used to make large, complex heavy-duty forging die.
Large-section hot forging die steels, 5Cr2NiMoVSi and 45Cr2NiMoVSi, have also been developed in China and have been widely used. Compared with 5CrNiMo steel, the carbon content of these steels is slightly lower, the content of Cr and Mo is increased and appropriate V and Si are added, resulting in high hardenability and thermal stability. Carbon and silicon in 45Cr2NiMoVSi steel are slightly lower than 5Cr2NiMoVSi steel and are more suitable for hammer forging dies. This kind of steel is used for making over 4000t mechanical press forging die and more than 3t forging hammer die, and the service life is increased by 0.5~1.5 times compared with 5CrNiMo and 5CrNiMoV. 3Cr2MoWVNi steel is also a kind of hot forging die steel developed in China and has a high service life.
2.2 Hot extrusion die steel H13 is widely used at home and abroad as hot work die steel. When the service temperature does not exceed 600°C, it has good hot and cold fatigue properties. It is used as hot extrusion die and aluminum alloy die casting die. A relatively high life. However, H13 steel has a large size effect, and foreign countries use external refining, high-temperature diffusion annealing, and other forging processes to improve the size effect and reduce the compositional segregation of Cr and Mo. Electroslag remelting and other processes are commonly used in China. .
China has developed many hot die steels for hot extrusion with good toughness and high thermal stability. Some steels are developed on the basis of foreign molybdenum-based 3Cr3Mo3V steels and chromium-based H13 steels, and have certain characteristics in alloying, such as HMI (3Cr3Mo3W2V), TM (4Cr3Mo2WMnVNb), Y4 (4Cr3Mo2MnVB), Y10 (4Cr5Mo2SiV1), HD2 (4Cr3Mo2VNiNbB), 012Al (5Cr4Mo3SiMnVAl) and other steels. These steels have high thermal stability under good toughness conditions and are used for making hot extrusion dies, precision forging dies, and non-ferrous metal die casting dies, respectively, and have good results.
Relevant departments in our country organized some research units and use units, selected 27 kinds of domestic and foreign applications and newly developed hot work die steels, tested and compared their basic mechanical properties, process performance and service performance, and proposed various types of Selection guidelines for hot work molds.
3 The output value of the mould for plastic mould steel plastic moulding has already taken the first place in the mould industry gross output value. In the past, China did not have steel for special plastic molds. In recent years, while introducing foreign plastic mold steel, it has also developed and developed some new special plastic mold steels.
3.1 Pre-hardened plastic mold steel This kind of steel is made into a module after being fully forged in a steel factory and pre-heat-treated to the required hardness (generally pre-hardened to 30~35 RHC) for use by the user. P20 (namely 3Cr2Mo) is the most widely used pre-hard plastic mold steel in foreign countries and has been listed as a standard for alloy tool steel in China. Since the 1980s, it has been widely used in some factories in China. 718 is a modified P20 steel produced in Sweden. It has higher hardenability than P20. After quenching and tempering, it can maintain uniform hardness in large section sizes, and it is also widely used in China.
3.2 Free-cutting pre-hardened steel In order to improve the cutting performance of pre-hardened plastic mold steel, free-cutting elements may be added. The United States, Japan, and Germany have all developed some free-cutting pre-hardened steels. The foreign easy-to-cut pre-hardened steels are mainly S series, but also S-Se series and Ca series. However, Se is more expensive. The anisotropy of S-cutting steel is large, and when the cross-section increases, the segregation of sulfide is more serious.
China has developed some sulfur-containing free-cutting pre-hardened plastic mold steels, such as 8Cr2MnWMoVS (8Cr2S) and S-Ca composite free-cutting plastic mold steel 5CrNiMnMoVSCa (5NiSCa). 5NiSCa steel uses the S-Ca compound free-cutting system and jet metallurgy technology to improve the morphology and distribution of sulfides and the anisotropy of the steel. The distribution of sulfides in the large section is still relatively uniform. 5NiSCa steel has high hardenability and mirror polishability. When the die hardness is 35~45HRC, various processes can be performed smoothly.
3.3 Non-tempered plastic mold steel This kind of steel can reach pre-hardness after forging and rolling without scheduling treatment, which is beneficial to energy conservation, cost reduction and shortened production cycle. The steels developed in China are: medium carbon manganese boron-based air-cooled bainite steel, which can be used to make plastic molds and rubber molds; non-tempered plastic mold steel 2Mn2CrVTiSCaRe (FT), with steel added S, Ca, and Re as Cutting elements have better cutting performance than S-Ca composite free-cutting steels; low-carbon MnMoVB non-tempered bainite-type large-section plastic die steels (B30), with S and Ca added as free cutting elements in steel, industrial The trial production showed that the 400 mm thick slab was air-cooled after hot rolling, and the hardness was evenly distributed along the section.
3.4 Age-hardening steels Several low-nickel age-hardening steels have been developed in China. These steels are machined after conditioning, and then aged to increase the hardness through the precipitation of intermetallic compounds. The deformation after heat treatment is very small. Age-hardening steel is suitable for making high-precision plastic molds, transparent plastic molds, and the like.
Such steels include 25CrNi3MoAl[3], 10Ni3Mn2AlCu(PMS) and 06Ni6CrMoVTiAl. After quenching and tempering, these steels have a hardness of 20~30HRC, can be machined, and then undergo aging, and the hardness can reach 38~42HRC.
3.5 plastic mold steel corrosion resistant plastic article during chemical corrosion plastic as raw material, the mold is required to have anti-corrosive properties, the general corrosion resistant steel molds, this time also requires good wear resistance. The commonly used steels are 4Cr13 (420), 9Cr18, and 17-4PH. PCR (0Cr16Ni4Cu3Nb) is an anti-corrosion plastic mold steel developed in China and has good corrosion resistance with good comprehensive mechanical properties.
4 Cemented Carbide and Steel-bonded Cemented Carbide Cemented carbide is a type of composite material manufactured by powder metallurgy. Carbide has high hardness, good wear resistance, high elastic modulus and high working temperature. For the production of certain molds, the service life of the mold can be increased several times or more than ten times. However, hard alloys are brittle, have poor bending strength and toughness, and cannot be machined. As a mold material, hard alloys are mainly used for drawing dies, cold extrusion and cold punching molds with little impact. At present, China has been able to produce various grades of hard alloys, which can basically meet the needs of the domestic market.
In order to meet the needs of manufacturing micro drill bits for drilling integrated circuit boards, dot matrix printing needles for computers, precision tooling, and the like, in recent years, various crystallites (WC grains less than 1 micron) and ultrafine crystals have been developed in various countries. Cemented carbide (WC grain less than 0.6 microns), the traditional carbide, WC grain size of 1.3 to 1.5 microns. Ultrafine grained carbides make up for many shortcomings of conventional cemented carbides, expand their application range, and achieve good results in the manufacture of wear-resistant and impact-resistant molds. Several research institutes and cemented carbide plants in China have developed various grades of microcrystalline hard alloys and ultrafine grained hard alloys. The development of high-performance ultrafine grained carbides is still a hot topic in the research of cemented carbides.
Steel cemented carbide is a composite material in which carbide is a hard phase and steel is a bonding phase. Steel-bonded carbide has good wear resistance, its strength and toughness are generally higher than that of cemented carbide, and has some process properties such as heat treatability, machinability, malleability, and weldability. The mold is the main application area of ​​steel-bonded carbide. China began to develop this kind of material in the 1960s and has developed various grades of steel cemented carbide as a steel cemented carbide for molds. The hard phase mainly uses TiC and WC, and the steel base mainly uses low-alloy chromium. Molybdenum steels, medium-high alloy tool steels or high-speed steels, such as TiC-based GT35, R5, D1, T1, and WC-based TLMW50, GW50, and GJW50. Steel-bonded carbide has been used to make cold dies, extrusion dies, drawing dies, blanking dies, wire drawing dies, hot dies, and the like.
The development of powder metallurgy technology and the application of hot isostatic pressing have led to the production and use of high-speed steel without segregation powder in the 1970s. Its main features are toughness, grindability, isotropy, and heat treatment processability. High-speed steel, and have a relatively high life. Later, this technology is used to produce high-carbon, high-vanadium, high wear-resistant cold die steels that cannot be produced by conventional processes. Such steels have good machinability and grinding performance, and have good toughness. Some hard alloys are similar. Abroad has produced a variety of brands of powder metallurgy high wear-resistant cold die steel, the domestic study is still less.
5 mold heat treatment mold manufacturing cost is high, especially some sophisticated and sophisticated cold stamping die, plastic die, die casting die and so on. The use of heat treatment technology to improve the performance of the mold, can greatly increase the life of the mold, there are significant economic benefits, China's mold technicians attach great importance to the development of mold heat treatment technology.
5.1 Vacuum Heat Treatment After the vacuum heat treatment, the die steel has a good surface state with small deformation. Compared with the quenching under the atmosphere, the hardening of the surface of the die after vacuum oil quenching is relatively uniform and slightly higher. The main reason is that during vacuum heating, the surface of the die steel is in an active state, does not decarburize, and does not produce an oxide film that hinders cooling. Heating under vacuum has a degassing effect on the surface of the steel and therefore has higher mechanical properties. The higher the degree of vacuum in the furnace, the higher the bending strength. After vacuum quenching, the fracture toughness of the steel is improved, and the mold life is generally increased by 40% to 400% or even higher than that of the conventional process. Vacuum die quenching technology has been more widely used.
5.2 Cryogenic treatment Recent research studies have shown that the deep cooling treatment (-196°C) of the die steel can improve its mechanical properties. Some of the die's deep cryogenic treatment significantly increase its service life. The deep cooling of the die steel can be performed between the quenching and tempering steps, and deep cryogenic treatment can also be performed after quenching and tempering. If residual austenite remains in the steel after quenching and tempering, a further tempering is still required after the cryogenic treatment. Cryogenic treatment can improve the wear resistance and anti-tempering stability of steel. Cryogenic treatment is not only used for cold working molds, but also for hot working molds and hard alloys. Cryogenic treatment technology has been increasingly concerned by mold heat treatment workers and special cryogenic processing equipment has been developed. The microstructure changes and microscopic mechanisms of different steel grades in the cryogenic process and their effects on mechanical properties need further study.
5.3 high temperature quenching and cooling quenching of some hot die steel, such as 3Cr2W8V, H13, 5CrNiMo, 5CrMnMo, etc., using higher than the conventional quenching temperature quenching, can reduce the number of carbides in the steel, improve its morphology and distribution, so that The distribution of carbon dissolved in austenite is even, and after quenching, more lath martensite can be obtained in the steel to improve its fracture toughness and cold and heat fatigue resistance, thereby prolonging the service life of the die. For example, a hot extrusion die made of 3Cr2W8V steel has a conventional quenching temperature of 1080 to 1120°C and a tempering temperature of 560 to 580°C. When the quenching temperature is increased to 1200°C and the tempering temperature is 680°C (2 times), the mold life is increased several times.
W6Mo5Cr4V2, W18Cr4V high-speed steel and Cr12MoV high alloy cold working die steel, can be appropriately reduced its quenching temperature, in order to improve its plastic toughness, reduce the tendency of brittle cracking, and thus improve die life. For example, the quenching temperature of W6Mo5Cr4V2 can choose 1140~1160°C.
5.4 Chemical Heat Treatment Chemical heat treatment can effectively improve the wear resistance, corrosion resistance, seizure resistance, and oxidation resistance of the mold surface. Almost all chemical heat treatment processes can be used for the surface treatment of mold steels.
Research shows that high-carbon and low-alloy tool steels and medium-high-carbon high-alloy steels can be carburized or carbonitrided. For carburizing or carbonitriding of high-carbon low-alloy steels, the lower heating temperature and shorter holding time should be selected as much as possible. At this time, more undissolved carbide cores, carburizing, and carbon and nitrogen can be ensured in the surface layer. After co-osmotic, the surface carbides are in the form of granules, and the total volume of carbides also increases significantly, which can increase the wear resistance of the steel. After the carburizing of W6Mo5Cr4V2 and 65Nb steel molds and vacuum carburizing of 65Nb steel molds, the service life of the molds has been significantly improved.
Alloy steel molds tempered at 500 to 650°C can be surface nitrided or nitrocarburized at temperatures below the tempering temperature or tempered.
Nitriding process currently uses ion nitriding, high frequency nitriding and other processes. Ion nitriding can shorten the nitriding time and obtain a high-quality penetration layer. Ion nitriding can improve the corrosion resistance, wear resistance, thermal fatigue resistance and anti-adhesion properties of die casting molds.
Nitrocarburizing can be carried out in a gas medium or a liquid medium, the seepage layer has a small brittleness, and the copres- sion time is much shorter than the nitriding time. The thermal fatigue performance of the die-casting mold and the hot-extrusion mold after nitrocarburizing can be significantly improved. Nitrocarburizing has good application effects on cold die, cold die, cold die, and tensile die.
Cold work molds and hot work molds can also be sulfur-nitrogen or nitrocarbo-nitrided. In recent years, many researches have shown that the rare earth has a significant effect of leaching, which led to the development of new processes such as rare earth nitrification and rare earth nitrocarburizing.
5.5 Boronizing and Boronizing Boronizing can be solid boronizing, liquid boronizing and paste boriding, etc. The most widely used is solid boronizing. The solid boronizing agent is available on the market. After solid boronization, the hardness of the surface layer is as high as 1400`2800HV, high wear resistance, and good corrosion resistance and oxidation resistance.
Boronizing process is often used on various cold working molds. Due to the improvement of wear resistance, the life of the mold can be increased by several times or more than ten times. The use of medium carbon steel boronization can sometimes replace the high alloy steel mold. Boronizing can also be applied to hot molds, such as hot extrusion dies.
The boronized layer is brittle, the diffusion layer is relatively thin, and the supporting force to the seepage layer is weak. For this reason, boron nitride or boron carbonitriding may be used to strengthen the transition zone and make the hardness change gentle. In order to improve the brittleness of the boronizing layer, boron-vanadium and boron-aluminum co-infiltration can be used.
The infiltration of metal, including chromizing, vanadium infiltration, titanium infiltration and other processes can be used to treat cold work and hot work die, in which TD method (molten salt infiltration metal) has been applied to make the life of the die several times or even more than ten times.
5.6 Vapor Deposition Vapor Deposition is divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD) according to the basic principle of formation.
PVD is classified into vacuum evaporation, sputtering, and ion plating. Ion plating is a combination of vapor deposition and sputtering. The ion plating has the advantages of strong adhesion, good throwing power, and good compatibility between the substrate material to be plated and the plating material, so it has been widely used. In recent years, multi-arc ion plating has attracted people's attention. At present, TiN is used for ion plating in molds. This film not only has high hardness but also has good toughness, strong bonding strength, and high temperature resistance. The multi-element membranes developed on the basis of TiN, such as (TiAl)N and (TiCr)N, are superior to TiN in terms of performance, and are a more promising new type of thin film.
CVD is a method of chemically reacting a reaction gas on the surface of a base material to form a coating layer (TiC, TiN). There are many ways to CVD. In general, the reaction temperature of CVD is 900° C. or more, and the coating hardness reaches 2000 HV or more. However, a high temperature tends to deform the workpiece and the reaction at the deposition layer interface is likely to occur. The trend is to lower the temperature and develop new coating compositions. For example, metal organic compound CVD (MOCVD), laser CVD (LCVD), plasma CVD (PCVD), and the like.
5.7 High Energy Beam Heat Treatment Heat sources for high energy beam heating usually refer to lasers, electron beams, ion beams, and the like. Their common feature is that the surface of the feed material has a power density of at least 103 W/cm2. Their common features are: heating speed, heating area can be selected according to need, the workpiece deformation is small, no cooling medium is needed, the processing environment is clean, the controllability is good, and it is easy to realize automatic processing. Domestic and foreign researches on the principle and process of high-energy beam heat treatment have all been devoted to research. The mature ones are laser transformation hardening, small-size electron beam processing and medium-power ion implantation, and they have been applied in improving the life of the die.
6 The outlook and recommendations can be considered that China has established a complete series of mold materials, some of which have excellent performance and reach the international advanced level. China's research and development of mold heat treatment can also be synchronized with the international, some new mold heat treatment technology has been promoted and applied to varying degrees.
In view of the existing problems, the following suggestions are made for the development of heat treatment technology for mold materials and molds in the future:
6.1 Accelerating the Productization of the Production of Die Steels, Fine Feeding, and Commercialization of Tool Steel Distributing Over 200,000 tons of steel are used annually in China, which has increased year by year. In recent years, the import volume of foreign mold steel, accounting for about 1/3 of the mold steel demand, has gradually increased. The main problem is that China's die steels have less specifications, and the product steels produced by the die steels have a low degree of commercialization, refinement, and commercialization of distribution. In some industrialized countries, metallurgical enterprises supply up to 50-60% of machined die steel products, while over 80% of China's die steel is still supplied with black bar. More and more mold manufacturers require that after the mold design is completed, the mold steel supplier can quickly provide the required steel , reduce the number of steel stocks, and shorten the molding cycle. China's steel production enterprises have not yet adapted to this commodity market mechanism. This is an important reason for the increasing import of mold steel in China.
6.2 Promote the application of new mold steels with excellent performance. Continuously improve the steel mold series of mold steel. China has developed a number of new mold steels with certain characteristics. Some of these steels have excellent performance and have reached or surpassed the level of similar foreign steels. However, the number and scope of application of these new steels are not large enough. The main reason is that due to the fact that China’s mold steel production has not yet gone through the path of productization and enrichment, and the distribution method does not meet the requirements of the commodity market, these problems are solved and these properties are good. The new type of mold steel has broad prospects for promotion and will produce huge economic benefits.
China has already had a relatively complete series of die steels, and it is still necessary to continuously improve its quality, expand its application, and make further improvements in applications. At the same time, we have selectively developed advanced die steels to improve China's die steel series, such as the development of powder metallurgy die steels, multiple free-cutting plastic die steels, and the establishment of glass, ceramics, firebricks, and floor tiles for forming die steels.
6.3 Further improve the quality of die steel Some special steel mills in China have adopted new metallurgical equipment and processes to produce die steels, such as out-furnace refining, vacuum smelting, quick forging machines and precision forging machines, etc. Some of the die steels have a great quality. Improve, such as D2, P20 and other steel have been exported, the quality of export products can reach the international advanced level. The industrialized countries have been trying hard to improve the purity, density, uniformity and quality stability of die steel. Some foreign companies have specified [O]10ppm, [H] 2ppm, and S50ppm in high-purity mold steels, because the further improvement of steel purity can significantly improve the toughness and fatigue properties of steel. For large molds, vacuum degassing and high-temperature diffusion annealing must also be adopted to reduce the segregation of alloying elements, and the isotropic forging process is used to increase the isotropy so that the transverse and thick-direction plasticity and toughness of the mold steel reach the longitudinal 80~ More than 90. Our country also needs to further work in this area.
6.4 Enhance the promotion and application of advanced mold heat treatment technology Controlled atmosphere heat treatment and vacuum heat treatment of molds should be further developed, popularized and applied. Some effective mold surface heat treatment technologies should improve their processes and strengthen their promotion and application. Enhance the manufacturing level of equipment and process materials, and strengthen the construction of heat treatment plants.