In the 1950s, China's die steel all followed forei

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In the 1950s, all die steel in China followed the foreign steel grades

into the 1960s, in order to save raw materials and improve the accuracy of blanks, less and no cutting technology and precision forming technology have developed rapidly. In order to improve production efficiency, many high-efficiency pressure processing equipment have been used, and the forging hammer has been gradually replaced by the press. The performance of the original die steel can not meet the high requirements of service conditions, which affects the service life of the die and the promotion and application of new pressure processing technology and equipment. In the late 1970s, the use of precision and large-scale engineering plastic products became increasingly widespread, and the demand for plastic mold steel increased sharply, which also put forward new requirements for the performance of plastic mold steel. At that time, there was no special steel for plastic mold in China

since the 1960s, with the support of relevant national ministries and commissions, Chinese scientific and technological workers have developed many new die steels in combination with national conditions. Some new steel grades with excellent service performance and good process performance have been welcomed by die manufacturers and users. During this period, some foreign steel grades were also introduced, some of which achieved good results through production trials [1]. For some cold working die steels and hot working die steels with good application effects, the relevant departments also organized performance comparison tests and studies respectively, and put forward suggestions for selection and application [2, 3]. In order to meet the needs of high wear-resistant and long-life molds, cemented carbides developed rapidly in China at the end of the 1950s. At the same time, a variety of steel bonded cemented carbides were developed and used as molds to achieve good results

this paper is divided into six parts: cold work die steel, hot work die steel, plastic die steel, cemented carbide and steel bonded cemented carbide, die heat treatment, outlook and suggestions

1 cold working die steel

at present, the commonly used cold working die steel in China is still the old steel grades of low alloy tool steel CrWMn and high carbon and high chromium tool steel Cr12MoV and Cr12. CrWMn steel has proper hardenability and wear resistance, and the deformation of heat treatment is small. However, CrWMn steel needs to strictly control the cooling rate after forging, and adopt appropriate heat treatment to make the carbides distributed on the matrix in uniform and fine particles, otherwise it is easy to form carbides, resulting in tool edge collapse and cracking in use. High carbon and high chromium tool steel has high wear resistance, but its carbide segregation is serious, which leads to the reduction of deformation directionality and strength toughness. The degree of segregation can be improved to a certain extent by repeated upsetting

In 1981, China introduced the internationally used high carbon and high chromium tool steel D2 (Cr12Mo1V1). Compared with Cr12MoV steel, the carbide segregation of D2 steel is slightly improved, and the strength and toughness are slightly improved. The service life of dies made of D2 steel is also improved to varying degrees [4]. High speed steel (mainly W6Mo5Cr4V2 and W18Cr4V) has higher wear resistance and strength, and is often used to make molds, but its toughness can not meet the needs of complex and large molds with high impact load

in order to improve the strength and toughness of this kind of steel, China has developed some new cold working die steels, such as:

1.1 low alloy cold working die steel

the main characteristics of this kind of steel are good processability, low quenching temperature, small heat treatment deformation, good strength and toughness, and appropriate wear resistance. Such as Gd (6crmnnimovsi), 7CrSiMnMoV (CH for short), DS steel, etc. GD steel is used to make cold stamping dies that are easy to collapse and fracture, and has a high service life [5]. The composition of CH steel is the same as that of sx105v steel in Japan. It is a kind of flame quenched steel, which is often used to make mold parts for production lines such as automobiles. During flame quenching, the cutting surface of the die edge is heated, and a high toughness matrix is used as a liner under the hardened layer, so that the die can obtain a higher service life [3[. DS steel is an impact cold working die steel, and its impact toughness is significantly better than the commonly used tool steel 6CrW2Si for high toughness blades [6]

1.2 matrix steel

matrix steel generally refers to the steel whose composition is the same as that of the matrix in the quenched structure of high-speed steel. In the early 1970s, the United States and Japan studied the matrix steel with grades of Vasco Ma, Vasco Matrix I and mod2, which is equivalent to the matrix of M2 and M36 high-speed steel, but it has not been widely used. China has developed some matrix steels, such as 65Cr4W3Mo2VNb (65Nb), 65w8cr4vti (LM1) and 65cr5mo3w2vsiti (LM2) steels. The main characteristics of these matrix steels are that their carbon content is slightly higher than that of the matrix, so as to increase the primary carbide content and improve the wear resistance. A small amount of strong carbide forming element niobium or titanium is also added to form relatively stable carbides, prevent the grain growth during quenching and heating, and improve the technological properties of the steel. This kind of matrix steel has been widely used to make dies for cold extrusion, cold punching of thick plates, cold heading, etc., especially for large and complex dies for difficult to deform materials, and also for warm extrusion dies of ferrous metals [7]

1.3 wear resistant cold working die steel with high toughness

in order to improve the carbide segregation of Cr12 cold working die steel, improve its toughness and further increase the wear resistance of steel, China has done a lot of research work and developed many new steel types, such as ld[8], er5[9] and gm[10] steel. In these steels, the chromium content is appropriately reduced to improve carbide segregation, and the contents of tungsten, molybdenum and vanadium are increased to increase the ability of secondary hardening and improve wear resistance. Compared with Cr12 cold working die steel, this kind of steel has improved carbide segregation and higher toughness. This kind of steel has better wear resistance than Cr12 cold working die steel, so the die made has a higher service life and is more suitable for the use of high-speed punches and multi station punches

2 hot work die steel

the commonly used hot work die steels in China are 5CrMnMo, 5CrNiMo and 3Cr2W8V steel

5CrNiMo steel is mainly used for large and medium-sized forging dies. However, its hardenability and tempering stability are not high enough, and its performance cannot meet the performance requirements of large section forging dies. 3Cr2W8V steel is widely used as hot extrusion dies for ferrous and non-ferrous metals and die-casting dies for Cu and Al alloys. This kind of steel has high thermal stability, with a service temperature of 650? C. However, tungsten series hot working die steel has low thermal conductivity and poor cold and hot fatigue

in the early 1980s, China introduced the foreign common chromium series hot work die steel H13 (4Cr5MoSiV1). H13 steel has good cold and hot fatigue, and the service temperature is not more than 600? C, replacing 3Cr2W8V steel, the service life of the die has been greatly improved, so H13 steel has been rapidly popularized and applied, and its output has exceeded 3Cr2W8V steel

in order to meet the higher requirements of new pressure processing technology and new equipment on the strength, toughness and thermal stability of die steel, China has developed many new hot work die steels, mainly:

2.1 hot forging die steel

in the 1980s, the hardenability of 5CrNiMo steel could not meet the needs of large section hammer forging dies, and the service temperature did not exceed 500? C, we have done a lot of analysis, comparison and Research on domestic and foreign steel. The research work shows that the contents of Cr, Ni and Mo in foreign similar steel 5crnimov are higher than domestic 5CrNiMo steel, and contain a small amount of V, so its hardenability and tempering stability are higher than domestic 5CrNiMo steel. It is suggested to choose 5crnimov steel to make large and complex heavy-duty hammer forging die [11]

large section hot forging die steels 5Cr2NiMoVSi and 45Cr2NiMoVSi have also been developed in China and have been widely used [1]. 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, so they have high hardenability and thermal stability. Compared with 5Cr2NiMoVSi steel, the carbon and silicon in 45Cr2NiMoVSi steel are slightly lower, which is more suitable for making hammer forging dies. This kind of steel is used to manufacture forging dies for mechanical presses of more than 4000t and forging hammer dies of more than 3T, and its service life is 0.5~1.5 times longer than that of 5CrNiMo and 5crnimov. 3Cr2MoWVNi steel is also a hot forging die steel developed in China, with high service life [11]

2.2 die steel for hot extrusion

h13 is a hot work die steel widely used at home and abroad, and its service temperature is not more than 600? C, it has good cold and hot fatigue performance. It is used as hot extrusion die and aluminum alloy die casting die, and has a relatively high service life. However, H13 steel has a large size effect. Processes such as out of furnace refining, high temperature diffusion annealing and isotropic forging are used abroad to improve its size effect and reduce the component segregation of Cr and Mo. electroslag remelting and other processes are mostly used in China

China has developed many hot die steels for hot extrusion with good strength, toughness and high thermal stability. Some steels are developed on the basis of foreign molybdenum 3cr3mo3v steel and chromium H13 steel, and have certain characteristics in alloying, such as HMI (3cr3mo3w2v), TM (4cr3mo2wmnvnb), Y4 (4cr3mo2mnvb), Y10 (4cr5mo2siv1), HD2 (4cr3mo2vninbb), 012AL (5cr4mo3simnval), etc. These steels have high thermal stability under the condition of maintaining good strength and toughness. They are used to make hot extrusion dies, precision forging dies, non-ferrous metal die-casting dies, etc., and have good application effects [3, 12]

the relevant departments of our country have organized some research units and user units to select 27 kinds of hot work die steels that have been successfully applied and newly developed at home and abroad, test and compare their basic mechanical properties, process properties and service properties, and put forward the selection criteria of various hot work dies [2]

3 plastic die steel

the output value of plastic forming dies has ranked first in the total output value of the die industry. In the past, China did not have steel for special plastic molds. In recent years, while introducing foreign steel for plastic mold, some new special steel for plastic mold have been developed by ourselves

3.1 pre hardened plastic mold steel

this kind of steel is made into modules after full forging in the steel factory, and is pre heat treated to the required hardness (generally pre hardened to 30~35rhc) for the use of unit mold making. P20 (i.e. 3Cr2Mo) is the most widely used pre hardened plastic mold steel abroad, which has been listed in the standard of alloy tool steel in China, and has been widely used in some factories in China since the 1980s. 718 is a modified P20 steel produced in Sweden, which has higher hardenability than P20. After quenching and tempering, it can maintain uniform hardness in large section size, and is also widely used in China

3.2 free cutting pre hardened steel [13]

in order to improve the machinability of pre hardened plastic mold steel, free cutting elements can be added. The United States, Japan and Germany have developed some free cutting pre hardened steels. Foreign free cutting pre hardened steels are mainly S-Series, but also s-se series and Ca series. But se is expensive. The anisotropy of S-Series free cutting steel is large. When the section increases, the sulfide segregation is serious, which is an important index to measure its electrical insulation ability

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 adopts S-Ca composite free cutting system and spray metallurgy technology, which improves the morphology and distribution of sulfide and the anisotropy of steel. The distribution of sulfide in large section is still relatively uniform. 5nisca steel has high hardenability and mirror polish. When the die hardness is 35~45hrc, various processing can be carried out smoothly

3.3 non quenched and tempered plastic die steel

this kind of steel can achieve pre hardening hardness after forging and rolling without scheduling treatment, which is conducive to saving energy, reducing cost and shortening production cycle. This kind of steel developed in China includes: medium carbon manganese boron air-cooled bainitic steel, which can be used to make plastic mold and rubber mold; Non quenched and tempered plastic die steel 2mn2crvtiscare (FT), with s, Ca and re as free cutting elements, has better cutting performance than S-Ca composite free cutting steel [13]; Low carbon mnmovb series non quenched and tempered bainite

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