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Research progress of mold materials and mold heat treatment

    2 Hot work die steel

    The commonly used hot work die steels in my country are 5CrMnMo, 5CrNiMo and 3Cr2W8V steels.

    5CrNiMo steel is mainly used for large and medium forging dies. But its hardenability is not high enough, tempering stability is not high, 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, and its use temperature reaches 650C, but the tungsten hot work die steel has low thermal conductivity and poor thermal fatigue.

    In the early 1980s, my country introduced the universal chromium-based hot work die steel H13 (4Cr5MoSiV1). H13 steel has good cold and hot fatigue properties. When the service temperature does not exceed 600C, it replaces 3Cr2W8V steel, and the die life is greatly improved. Therefore, H13 steel has been quickly promoted 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 and toughness and thermal stability of die steel, my country has developed many new hot work die steels, mainly including:

    2.1 Hot forging die steel

    In the 1980s, domestic and foreign steels were analyzed, compared and researched on the problem that 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. Research work shows that the contents of Cr, Ni, Mo in similar foreign 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, and 5CrNiMoV is recommended. Steel, used to make large and complex heavy-duty hammer forging dies [11].

    Large cross-section hot forging die steel 5Cr2NiMoVSi and 45Cr2NiMoVSi steel have also been developed in China, which have been widely used [1]. Compared with 5CrNiMo steel, the carbon content in these steels is slightly lower, the content of Cr and Mo is increased and the appropriate V and Si are added, because of its high hardenability and thermal stability. Compared with 5Cr2NiMoVSi steel, the carbon and silicon in 45Cr2NiMoVSi steel are slightly lower, and it is more suitable for hammer forging die. This kind of steel is used to manufacture forging dies for mechanical presses above 4000t and forging hammer dies above 3t. The service life is 0.5″1.5 times longer than that of 5CrNiMo and 5CrNiMoV. 3Cr2MoWVNi steel is also a kind of steel for hot forging dies developed in my country and has high usage Life [11].

    2.2 Die steel for hot extrusion

    H13 is a hot work die steel widely used at home and abroad. When the service temperature does not exceed 600C, it has good cold and hot fatigue properties. It is used as a hot extrusion die and aluminum alloy die-casting die and has a relatively long service life. However, H13 steel has a larger size effect. In foreign countries, processes such as furnace scouring, high temperature diffusion annealing, and isotropic forging are used to improve its size effect and reduce the segregation of Cr and Mo components. In China, electroslag remelting is often used. .

    my country 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 series 3Cr3Mo3V steel and chromium series H13 steel, 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 while 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 results [3, 12] .

    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 applied and newly developed at home and abroad, and tested and compared their basic mechanical properties, process properties and service properties, and proposed various types Selection criteria for hot work molds [2].

    3 Plastic mold steel

    The output value of molds for plastic molding has taken the first place in the total output value of the mold industry. China used to have no special steel for plastic molds. In recent years, while introducing foreign steel for plastic molds, some new special steels for plastic molds have been independently researched and developed.