Generally, the selection of steel grades can be divided into the following four conditions for the use of cold working dies:
(1) Cold working die with small size, simple shape and light load. For example, small punch, scissors for cutting steel plate can be made of t7a, T8A, T10A, T12A and other carbon tool steels. This kind of steel has the advantages of good machinability, low price and easy source. But its disadvantages are: low hardenability, poor wear resistance, quenching deformation. Therefore, it is only suitable for making tools with small size, simple shape and light load, as well as cold image mould with low hardening layer and high toughness.
(2) Cold working die with large size, complex shape and light load. The commonly used steels are 9SiCr, CrWMn, GCr15 and 9mn2v low alloy cutting tool steels. The quenching diameter of these steels in oil is about 40mm. In our country, 9Cr steel can be used instead of 9V steel.
9mn2v steel has smaller carbide heterogeneity and quenching cracking tendency than CrWMn steel, less decarburization tendency than 9SiCr Steel, and its hardenability is higher than that of carbon tool steel, and its price is only about 30% higher than that of the latter. Therefore, 9mn2v steel is worth popularizing.
However, 9mn2v steel also has some shortcomings, such as low impact toughness and fragmentation phenomenon found in production and use. In addition, the tempering stability is poor, the tempering temperature is generally not more than 180 ℃, and the bending strength and toughness begin to appear low values at 200 ℃.
9mn2v steel can be quenched in medium with mild cooling capacity such as nitrate and hot oil. Austenite austempering can be used for some dies with strict deformation requirements and low hardness requirements.
(3) Cold working die with large size, complex shape and heavy load. Medium alloy or high alloy steel, such as cr12mo, Crl2MoV, cr6wvcr4w2mov, etc., must be used. In addition, high speed steel is also selected.
The tendency of using high speed steel as cold working die is increasing day by day. However, it should be pointed out that at this time, it is no longer taking advantage of the unique red hardness of high speed steel, but using its high hardenability and high wear resistance. Therefore, the heat treatment process should be different.
When using high speed steel as cold die, low temperature quenching should be adopted to improve toughness. For example, the quenching temperature of W18Cr4V Steel is 1280-1290 ℃. The cold working die should be quenched at 1190 ℃. For example, the service life of W6Mo5Cr4V2 steel can be greatly improved after low temperature quenching, especially the reduction rate.
(4) cold working die with impact load and thin between tools. As mentioned above, the performance requirements of the first three types of cold working die steel are mainly high wear resistance, so high carbon hypereutectoid steel and even rongite steel are used. But for some cold working dies, such as adding trimming floor and blanking die, the counterpart is thin. When it is used under impact load, high impact toughness should be the main requirement. In order to solve this contradiction, the following measures can be taken: 1) reduce the carbon content and adopt hypoeutectic steel to avoid the decrease of toughness due to primary and secondary carbides; 2) adding Si, Cr and other alloy elements to improve the tempering stability and tempering temperature (240-270 ℃). This is conducive to fully eliminating the quenching stress and increasing the hardness without reducing the hardness; 2 In order to refine grain size and improve toughness, W and other elements are added to form refractory carbide. The commonly used high toughness cold working die steels are 6sicr, 4crw2si, 5CrW2Si, etc.