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Research progress of mold materials and mold heat treatment

    In the 1950s, all the steel used for molds in my country followed foreign steel grades. In the 1960s, in order to save raw materials and improve the precision of blanks, the cutting process and precision forming technology have developed rapidly. In order to improve production efficiency, many high-efficiency pressure processing equipment are used, and forging hammers are gradually replaced by presses. The performance of the original mold steel often cannot meet the high performance requirements of the service conditions, which affects the service life of the mold and the promotion and application of new pressure processing technology and new equipment. At the end of the 1970s, the use of precision and large-scale engineering plastic products became more and more widespread, and the demand for plastic mold steel increased sharply. New requirements were also put forward for the performance of plastic mold steel. However, there is no special steel for plastic molds in my country.
    Since the 1960s, with the support of relevant national ministries and commissions, Chinese scientific and technological workers have developed many new mold steels in accordance with national conditions. Some of the new steels with excellent performance and process performance have been favored by mold manufacturers and users. welcome. During this period, some commonly used foreign steel grades were also introduced, and some steel grades have been tried in production and achieved good results [1]. For some cold-work die steels and hot-work die steels with better results, the relevant departments have also organized performance comparison test studies, and put forward suggestions for selection and application [2, 3]. In order to meet the needs of high wear-resistant, long-life molds, my country’s cemented carbide has developed rapidly in the late 1950s, and a variety of steel-bonded cemented carbides have also been developed to achieve good results when used as molds.

    This article 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, prospects and recommendations.

    1 Cold work die steel

    At present, the commonly used cold work die steels in our country are still low alloy tool steel CrWMn and high carbon and high chromium tool steel Cr12MoV and Cr12. CrWMn steel has proper hardenability and wear resistance, heat treatment deformation is small, but after forging CrWMn steel requires strict control of the cooling rate, and appropriate heat treatment, so that the carbides are uniform and fine grained and distributed on the substrate. Otherwise, it is easy to form network carbides, leading to chipping and cracking of the mold in use. High carbon and high chromium tool steel has high wear resistance, but its carbide segregation is serious, resulting in a decrease in the directionality of deformation and strength and toughness. The degree of segregation can be improved to a certain extent by repeated upsetting.

    In 1981, my country 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 compared to Cr12MoV, and the strength and toughness are slightly improved. The service life of the mold 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 cannot meet the needs of complex, large and impact-loaded molds.

    In order to improve the strength and toughness of this type of steel, my country has developed some new cold work die steels, such as:

    1.1 Low alloy cold work die steel

    The main characteristics of this type of steel are good manufacturability, low quenching temperature, small heat treatment deformation, good strength and toughness, and appropriate wear resistance. Such as GD (6CrMnNiMoVSi), 7CrSiMnMoV (abbreviated as CH), DS steel, etc. GD steel is used to make cold punching dies that are easy to chip and break and have a long service life [5]. The composition of CH steel is the same as that of SX105V steel in Japan. It is a flame-quenched steel. It is often used to make mold parts in automobile production lines. During flame quenching, the cutting surface of the die edge is heated, and there is a high toughness under the hardened layer. The base body is used as a liner, so that the mold can obtain a higher service life [3[. DS steel is a kind of impact cold work die steel, its impact toughness is significantly better than the commonly used tool steel for high toughness blade 6CrW2Si [6].

    1.2 Base steel

    The matrix steel generally refers to the steel whose composition is the same as the matrix chemical composition in the quenched structure of high-speed steel. The United States and Japan have studied the base steels of Vasco MA, Vasco Matrix I and MOD2 in the early 1970s, which are equivalent to the base of M2 and M36 high-speed steel, but they have not been widely used. my country has developed some base steels, such as 65Cr4W3Mo2VNb (65Nb for short), 65W8Cr4VTi (LM1 for short), 65Cr5Mo3W2VSiTi (LM2 for short) steel, etc. The main feature of these base steels is that their carbon content is slightly higher than that of the base to increase the amount of primary carbides and improve wear resistance. A small amount of strong carbide forming elements niobium or titanium are also added to form a relatively stable carbonization. It prevents the growth of grains during quenching and heating and improves the process performance of steel. This kind of base steel has been widely used to make molds for cold extrusion, cold punching of thick plates, cold heading, etc. It is especially suitable for large and complex molds for difficult-to-deform materials, and it can also be used as warm extrusion molds for ferrous metals [7 ].

    1.3 Wear-resistant cold work die steel with higher toughness

    In order to improve the carbide segregation of Cr12 cold work die steel, increase its toughness, and further increase the wear resistance of steel, my country has done a lot of research work and developed many new steel grades, such as LD[8], ER5 [9] and GM [10] steel, etc. In these steels, the content of chromium is appropriately reduced to improve carbide segregation, and the content of tungsten, molybdenum and vanadium is increased to increase the ability of secondary hardening and improve wear resistance. Compared with Cr12 type cold work die steel, this type of steel has improved carbide segregation and higher toughness. This kind of steel has better wear resistance than Cr12 cold work die steel, so the made mold has a higher life and is more suitable for high-speed punching and multi-station punching.