(2) Low-carbon high-speed steel is obtained by reducing the wC of high-speed steel to 0.3% to 0.6%, which can reduce the amount of eutectic carbides, maintain high red hardness, and improve the toughness of steel. Thermal fatigue properties, such as H25, H26 and H42 in the United States.

(3) High-temperature hot work die steel For martensite-based hot work die steel, this kind of steel has poor thermal conductivity, large linear expansion coefficient and poor thermal fatigue. It is not suitable for high temperature molds under chilled and hot conditions. Such as Japan's 5Mn15Cr10V2.

(4) High-temperature corrosion-resistant mold steel can improve the erosion resistance and high-temperature oxidation resistance of molds at high temperatures against liquid alloys and other media, and is used to make molds for die-casting molds and pressed glass. Such as 3Cr13MoV and so on.

(5) High-hardenability hot work die steel If it is suitable for steel for extra large mold modules, 4NiCr2MoV developed on the basis of 5CrNiMoV; 40NCD16 (4Ni4Cr2Mo) in France NF35590 standard; 4Cr2MoVNi in China.

(6) Medium alloy high toughness hot work die steel This kind of steel can use alloying elements reasonably and reduce the production cost of the product, so it has developed rapidly in recent years. By reducing the chromium content on the basis of H13 steel and increasing the vanadium content, it is developed into a steel with MC-type carbide as the main strengthening phase. Representative is Sweden's QRO80.

3. Cast iron materials are also widely used

Automotive surface covering casting die is commonly used in cast iron materials, such as HT300, QT600-2, Mo, MoCr and MoV, Japan's TGC600, FCD250, FCD540, and GGG70L commonly used in German cars.

Second, the latest development of automotive mold heat treatment

The mold material has been greatly developed, and the heat treatment technology of the mold has also developed rapidly. The heat treatment of the mold mainly has the following applications.

1. Overall heat treatment technology

Most molds require integral heat treatment, such as die-casting, forging, cold boring, and partial dies. The overall heat treatment equipment includes a salt bath furnace, a box furnace and a vacuum furnace. Select the appropriate heat treatment equipment according to the application and size of the mold. Generally, the molds are small in size, and the molds that do not allow oxidative decarburization on the surface are often heated and quenched by a salt bath furnace; the larger the size, the less important, and the molds that can be hot post-processed are mostly quenched by box furnace; high alloy steel Most use vacuum quenching. The box furnace should have a good sealing property. In order to prevent oxidative decarburization, a protective gas can be introduced into the furnace. In addition, the workpiece can be coated with an anti-oxidation decarburization coating and some charcoal can be placed in the furnace during heat treatment. In order to prevent workpiece deformation and oxidative decarburization, vacuum heat treatment furnaces are used in all countries to treat molds, and vacuum heat treatment can also improve the service life of molds. Vacuum heat treatment is divided into gas quenching and oil quenching. Oil quenching is much larger than gas quenching, and the mold with large machining allowance after heat treatment is suitable for oil quenching. When using a vacuum heat treatment gas quenching furnace, different quenching gas pressures can obtain different heat treatment deformations. The larger the pressure, the larger the deformation, the smaller the pressure, the smaller the deformation, and the different parameters should be selected according to different workpieces.

2. Surface hardening

Surface quenching is suitable for occasions where the deformation is small, the workpiece size is large, and the rest is auxiliary. Most of the automobile cover moulds are relatively large, and there are many surface quenching. Among them, the molds of air-cooled steel, cast iron and alloy cast iron are mostly surface-hardened. Surface quenching methods are also different, with flame quenching, induction hardening and laser quenching.

Flame quenching can be divided into acetylene flame quenching and propane flame quenching according to the gas used. Propane flame quenching can be used with water to reduce mold deformation and increase mold hardness.

Induction hardening is mostly used for molds ordered by German joint ventures. German molds require surface quenching for induction heating and quenching, using a super-audio heating power supply of 10 to 50 kHz. It has the advantage of deepening the heating layer. Generally, the depth of the hardened layer with the same surface hardness is about 2 mm. The disadvantage is that the deformation is large and cracking is easy. At present, domestic mold manufacturing companies mostly use induction hardening equipment produced by Norwegian EFD company. The equipment is stable and reliable, with power of 18~25kW and some 50kW. The higher the power, the higher the efficiency, but it is difficult to control; the larger the power, the smaller the deformation, the smaller the tendency to crack, but it is easy to burn.

Laser quenching is a surface quenching technique developed in recent years. It has the characteristics of thin hardened layer, high hardness, good toughness and position determination. The biggest advantage is that the mold is less deformed after quenching, and the quenching can be repeated several times. The shortcoming is that the equipment investment is large, the domestic complete equipment is about 2 million yuan, the efficiency is not very high, it is much slower than induction hardening, and the price is also high. The power of laser quenching is generally above 3 000 W, mostly 5,000 W.

The surface quenching characteristics of the above three processes are shown in Table 4.

3. Cryogenic treatment

After deep cooling treatment (-110 ~ -196 °C), the die steel can improve its mechanical properties, and some molds have significantly improved the service life after cryogenic treatment. The cryogenic treatment of the die steel can be carried out between the quenching and tempering processes, or after quenching and tempering. If retained austenite remains in the steel after quenching and tempering, a further tempering is required after the cryogenic treatment. In addition, cryogenic treatment can also improve the wear resistance of steel. Cryogenic treatment is not only used for cold work dies, but also for hot work dies and hard alloys. Cryogenic treatment technology has received increasing attention from mold heat treatment workers, and specialized cryogenic treatment equipment has been developed. The results of the microstructure changes and microscopic mechanisms of different steel grades during deep crying, as well as the effects on mechanical properties, are different. The different geometries and different states of use of the same material have different lifetimes after cryogenic treatment.

4. Surface coating technology

Vapor deposition is divided into chemical vapor deposition (CVD) and physical vapor deposition (PVD) according to the basic principles of formation. The CVD treatment is a method in which a reaction gas is chemically reacted on the surface of a base material to form a coating layer (TiC, TiN) by a chemical method. There are various methods for CVD. Generally, the reaction temperature of CVD is >900 ° C, and the coating hardness is >2000 HV. However, the high temperature tends to deform the workpiece, and the interface of the deposited layer is liable to react. The trend in CVD is to lower the temperature and develop new coating compositions. For example, metal organic compound CVD (MOCVD), laser CVD (LCVD), and plasma CVD (PCVD). PVD is a physical vapor deposition, generally with TiN coating, in addition to TiC, TiCN, TiAlN, CrN, Cr7C3 and W2C coatings. The layer depth is about 5mm, which does not affect the precision of the mold.

The surface coating technique uses oxygen acetylene to spray a substance onto the surface of the mold. The combination of the coating and the substrate is strong, which can increase the surface thickness and increase the wear resistance.

Die local surfacing technology. Many body blanking dies require a good toughness of the cutting edge. The use of integral die steel is relatively wasteful, and the method of stacking the cutting edge on a lower material can save a lot of steel and reduce production costs. The development of the United States, Japan and Germany in this respect is relatively mature, and China's mold surfacing has also made great progress.

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