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1. Working conditions and performance requirements of cold work die steel
Cold work mold steel at work. Due to the large deformation resistance of the processed material, the working part of the mold is subjected to great pressure, bending force, impact force and friction force. Therefore, the normal scrapping of cold work tools is generally caused by wear and tear. There is also premature failure due to fracture, collapse and deformation.
Cold work mold steel compared with cutting tool steel. There are many things in common. The mold is required to have high hardness and wear resistance, high bending strength and sufficient toughness to ensure the smooth progress of the stamping process. The difference lies in the complexity of the mold shape and the addition of the I process. And the friction area is large. It is highly wearable. So it is difficult to repair. Therefore, it is required to have a higher wear-resisting die to withstand high pressure when working. Because the complex shape is easy to produce stress concentration, it requires higher toughness; the size of the die is large and the shape is complex. Therefore, higher hardenability, less deformation and cracking tendency are required. In short, the requirements of cold work tool steel in hardenability, wear resistance and toughness are higher than cutting tool steel. In the case of red hardness, it is required to be low or basically not required (because it is cold forming), so steels suitable for cold work are also correspondingly formed, for example, high wear-resisting, micro-deformation cold is developed. Die steel and high toughness cold die steel. The following description will be combined with the selection of relevant steels.
2. The steel type selection is usually connected to the use conditions of cold molds. The steel types can be divided into the following four conditions:
(1) Cold working mold with small size, simple shape and light load. E.g. Small punches, scissors that cut off steel plates can be made of carbon tool steels such as T7A, T8A, T10A, and T12A. The advantages of this kind of steel are: good workability, low price, and easy source. However, its disadvantages are: low hardenability, poor wear resistance, and large quenching distortion. Therefore, it is only suitable for the manufacture of tools that are small in size, simple in shape, light in weight, and cold image molds that require a hardened layer that is not deep and maintains high toughness.
(2) Large-sized, complex-shaped, light-loaded cold-working molds. The commonly used steel grades include 9SiCr, CrWMn, GCr15, and 9Mn2V low alloy cutting tool steels. The hardened diameter of these steels in oil can generally be more than 40mm. Among them, 9Mn2V steel is a non-Cr cold working mold steel developed in recent years in China. The Cr-containing steel can be replaced or partially replaced.
9Mn2V steel carbide inhomogeneity and quenching cracking tendency than CrWMn steel, decarbonization tendency than 9SiCr steel, and hardenability than carbon tool steel. Its price is only about 30% higher than the latter, so it is a worthy steel to use. However, 9Mn2V steel also has some shortcomings such as low impact toughness, found in the production and use of cracking phenomenon. In addition, the tempering stability is poor, and the tempering temperature generally does not exceed 180° C. When tempered at 200° C., the flexural strength and toughness begin to appear low values.
9Mn2V steel can be quenched in quenching media, such as nitrates, hot oils, etc., with moderate cooling capacity. For some molds with strict deformation requirements and low hardness requirements, austenite austempering can be used.
(3) Large-size, complex-shaped heavy-duty cold-working molds. Must use alloy or high alloy steel. Such as Cr12Mo, Crl2MoV, Cr6WV, Cr4W2MoV, etc., there are also selected high-speed steel.
In recent years, the tendency of high-speed steel to make cold-work molds has increased day by day, but it should be pointed out that this is no longer the use of high-speed steel unique red strength. With its high hardenability and high wear resistance. to this end. There should also be differences in the heat treatment process.
When using high speed steel for cold molds. Should use low temperature quenching. To improve toughness. For example, when W18Cr4V steel is used as a cutting tool, the commonly used quenching temperature is 1280-1290°C. For cold molds, low temperature quenching at 1190°C should be used. Another example is W6Mo5Cr4V2 steel. The use of low-temperature quenching can greatly increase the service life, and in particular significantly reduce the breakage rate.
(4) Cold working molds with thin impact between the blades due to impact load. As mentioned above. The performance requirements of the first three types of cold-working die steels are all based on high wear resistance. For this purpose, high-carbon hypereutectoid steels and even Wyeth steels are used. And on some cold molds plus cutting edge building, blanking die and so on. Its counterpart is thin. When used again under the impact of the impact load should be based on the requirements of high impact toughness. In order to resolve this contradiction. Take the following measures. 1 reduce the amount of carbon. The use of sub-composite steel. To avoid due to primary and secondary carbides caused by the decline in the toughness of the steel; 2 add Si. , Cr and other alloy elements. In order to improve the steel tempering stability and tempering temperature (240-270 °C tempering) this is conducive to the full elimination of the quenching stress to improve the å½. And does not reduce the hardness; 2 Add W and other elements that form refractory carbides to refine the grain and improve toughness. The commonly used high-toughness cold working die steels include 6SiCr, 4CrW2Si, and 5CrW2Si.
3. Give full play to the potential of cold work die steel
When using Cr12 steel or high speed steel as cold die, a very prominent problem is the brittleness of steel. Easy to crack in use. To do this, the carbide must be refined with a full forging method. In addition, new steel grades should be developed. The focus of developing new steel grades should be to reduce the carbon content of the steel and the amount of carbide forming elements. In recent years, the following new steel types have been developed and promoted, as shown in Table 4.11. Cr4W2MoV steel has the advantages of high hardness, high wear resistance and good hardenability. And has a good temper stability and comprehensive mechanical properties. With dry manufacturing silicon steel die, and so on. It can increase the life of Cr12MoV steel by 1~3 times, but the scope of this steel forging temperature zone is narrow, and the forging river county cracks. Strict control of forging temperature and operation should be considered. Cr2Mn2SiWMoV steel quenching temperature is low, quenching deformation is small, high hardenability. It is called 7W7Cr4MoV steel which can be used for quenching and micro-deformation of die steel. It can replace W18Cr4V and Cr12MoV steel. It is characterized by a great improvement in the non-uniformity and toughness of steel carbides.
Second, hot die steel
1. Working conditions of hot work molds Hot work dies include hammer forging dies, hot extruding dies and die casting dies. As mentioned before. The main feature of working conditions of hot work molds is the contact with the hot metal, which is the main difference with the working conditions of the cold work mold. Therefore, it will bring about the following two problems:
(l) The cavity surface metal is heated. Usually hammer forging work. The cavity surface temperature can reach 300~400°C and the hot extrusion die can reach 500~800°C; the temperature of the die cavity is related to the type of die-casting material and the pouring temperature. If die casting ferrous metal mold cavity temperature up to 1000 °C. Such a high use temperature will significantly reduce the surface hardness and strength of the cavity, and will cause snoring during use. to this end. The basic performance requirements for hot die steels are high resistance to thermoplastic transformation, including high temperature hardness and high temperature strength, high resistance to thermoplastic transformation, in fact reflecting the high tempering stability of steel. From this, we can find the first way of hot die steel alloying, adding Cr, W, Si. Other alloying elements can improve the tempering stability of the steel.
(2) Thermal fatigue (cracks) occurs in the surface layer of the mold cavity. Hot die features are intermittent. Each time the hot metal is shaped, the surface of the mold cavity is cooled with water, oil, air, and the like. therefore. The working state of the hot mold is repeated heating and cooling, so that the surface metal of the mold cavity generates repeated thermal expansion and contraction, that is, it is repeatedly subjected to tensile and compressive stress. As a result, cracks appear on the surface of the mold cavity, which is called thermal fatigue. Therefore, a second basic performance requirement for hot mold steel is proposed. That is, it has high thermal fatigue resistance. In general, the main factors influencing the thermal fatigue resistance of steel are:
1 The thermal conductivity of steel. The high thermal conductivity of steel can reduce the degree of heating of the surface metal of the mold, thereby reducing the thermal fatigue tendency of the steel. It is generally believed that the thermal conductivity of steel is related to the amount of combined carbon. When the carbon content is high, the thermal conductivity is low. Therefore, high-carbon steel should not be used for hot die steel. In production, medium carbon steel (C0.3% 5~0.6%) is usually used to reduce the carbon content. Will cause the hardness and strength of steel to drop. It is also unfavorable.
2 The critical point effect of steel. Usually the higher the critical point (Acl) of steel. The lower the thermal fatigue tendency of steel. therefore. The critical point of the steel is generally increased by adding alloying elements Cr, W, Si, and lead. In order to improve the thermal fatigue resistance of steel.
2. Common hot die steel (1) Forging die steel. Generally speaking, the hammer forging die steel has two problems that are more prominent. It is affected by the impact load during work. Therefore, the mechanical properties of steel are required to be higher, especially for plastic deformation resistance and toughness; second, the cross-sectional size of the hammer forging die is larger (<400 mm), so the hardenability of the steel is required to ensure the entire Mold organization and performance are even.
The commonly used hammer forging floor steel is 5CrNiMo, 5CrMnMo, 5CrNiW, 5CrNiTi and 5CrMnMoSiV. Different types of hammer eye molds should use different materials. For large or large hammer forging die with 5CrNiMo as well. 5CrNiTi, 5CrNiW or 5CrMnMoSi can also be used. For small and medium sized hammer forging dies, 5CrMnMO steel is usually used.
(2) Hot Extrusion Die Steels, Hot Extrusion Dies are characterized by slower loading speeds and, therefore, higher temperatures in the die cavity, typically up to 500-800°C. The performance requirements for such steels should be based on high high temperature strength (ie high tempering stability) and high thermal fatigue resistance. The AK and hardenability requirements can be appropriately lowered. General hot extrusion die size is small, often less than 70 ~ 90 mm.
Commonly used hot extrusion die are 4CrW2Si, 3Cr2W8V and 5% Cr hot die steel. The chemical composition is shown in Table 4.16. Among them 4CrW2Si. Can do cold work die steel, but also do hot work die steel. Due to different uses, different heat treatment methods can be used. For cold molds, lower quench temperatures (870-900°C) and low or medium temper tempering are used; for hot molds, higher quench temperatures (typically 950-1000°C) and high temperature tempering are used.
(3) Steel for die-casting molds. In general, the performance requirements for steels for die casting applications are similar to those for hot extrusion die steels, that is, they are mainly required for high tempering stability and high thermal fatigue resistance. Therefore, the steel type that is usually selected is generally the same as the hot extrusion die steel. As usual 4CrW2Si. And 3Cr2W8V steel. But it is different, such as the lower melting point Zn alloy die-casting mold. 40Cr, 30CrMnSi and 40CrMo are optional; for Al and Mg alloy die-casting molds, Cu alloy die-casting molds such as 4CrW2Si and 4Cr5MoSiV are available. More use of 3Cr2W8V steel.
In recent years. With the application of black metal die casting process, high melting point aluminum alloys and nickel alloys are often used. Or 3Cr2W8V steel Cr-Al-SI ternary co-permeability for the manufacture of ferrous metal die casting. Recently, high-strength copper alloys are also being tested at home and abroad as die casting materials for ferrous metals.
Difference between cold die steel and hot die steel components
First, cold work die steel Cold work die steel includes cutting die (blank punching die, trimming die, punch, scissors), cold die and cold extrusion die, bending die and wire drawing die, etc.