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    Material selection and heat treatment of cold deformed die s
    With the development of mechanical processing industry, more and more components rely on cold working dies in production, and more and more cold deformation moulds are used. Cold volume die forging (cold heading, cold extrusion, imprint, etc.); sheet stamping (such as drawing, blanking, trimming, punching, etc.); material rolling (cold rolling, wheel forming, etc.). Although there are many kinds of cold deformed dies, the working conditions are different, the performance requirements are different, but the basic working conditions are similar: that is, the metal is deformed in the cold state, and the work bears great shear force, pressure, bending force, impact force and friction force.

    Therefore, the die steel should have the following basic properties:

    1) high hardness and high strength to ensure that it is not easy to produce micro plastic deformation or failure under high stress.

    2) high wear resistance, which ensures the dimensional accuracy of the die under high wear conditions. This is more important for drawing dies and cold stamping dies.

    3) enough toughness to avoid hard working spalling or collapse under impact load and dynamic load.

    4) the deformation of heat treatment is small. Because most dies have complex cavity and precision, the deformation of heat treatment is difficult to be eliminated by grinding and grinding. In addition, some cold deformed dies should have enough heat resistance. How to reasonably choose the material of the cold deformation die and the proper heat treatment means to improve the service life of the die is the most important problem in production. As the normal failure of the cold deformed die, it should be wear, micro plastic deformation and fatigue failure. But in actual production, in addition to grinding loss effect, and collapse, tenacity fracture, cracking and other premature failure, is common. It is well known that the life of the die with cold deformation is a comprehensive factor, the rationality of the selection of raw materials, the metallurgical quality in the raw material, the rationality of the design and manufacturing process, the conditions of use and the technical level of the workers.

    1. The problem of material selection for cold deformed die steel
    The following factors should be considered in selecting cold deformation die.

    1) the working conditions of mold, including equipment type, processing frequency, processed material, lubrication and environment.

    2) the complexity of the mold shape and the processing procedure;

    3) the production batch and technical requirements of the product;

    4) the difficulty and processing condition of the material source of the mold, the material cost of the large mold often accounts for more than 50% of the die cost. For the mold with complex shape and high precision, the mould steel with small quenching deformation should be selected.
    According to actual use, the common cold deformed die steel has low hardenability steel, low deformation steel, micro deformation steel, high strength steel, high toughness steel, and impact cold deformation die steel.

    2, the characteristics and processing technology of various cold deformed die steels

    2.1 cold deformable die steel with low hardenability
    This kind of steel is mainly carbon tool steel, especially high carbon steel GCr15 and 9Cr2Mo, among which GCr15 bearing steel and 9Cr2Mo are used to make various forming rollers and cold rolls as the representative. GCr15 steel has good wear resistance, but the hardenability is poor and the quenching deformation is large. As a small cold roll ([Phi < < 200m/m), the whole heating or whole height can be used. Temperature 900 - 950 C fast heating, quenching and cooling with salt water and oil, control the cooling time of the workpiece in brine and then tempering at 200 - 220 degrees C, the hardness of the roller surface is controlled between Rc60 - 62, and can get a higher service life. If the hardness is more than 62 or more than 62, the surface of the roll surface can be spalling when the surface of the roll is rolled, and the GCr15 is used. The forming rolling wheel can be quenched by isothermal quenching by alkali bath or with the solution of nitrite and three nitrate to control the cooling time in the solution. The effect of high hardness and small deformation on the rolling wheel is achieved by taking out the air cooling or oil cooling.
    With the development of the processing industry and the development of material transformation industry, the corresponding high speed and multi roll rolling steel 9Cr2Mo and the German roll steel 86CrMoV7 are necessary in the industrial method.
    We need to make new breakthroughs, make efforts in strengthening and toughening treatment, and make new breakthroughs in the adoption of new roller steels. At present, there are D2 (Cr12MOv, Cr12Movco) and 5H12 (5Cr5MovSi) for working rolls of high speed mills. The material of work roll should be selected according to the viewpoint of contact extrusion fatigue stress and friction wear, and the material of transmission roller should be selected with the ability of accident resistance.

    2.2 low deformation and deformation die steel

    1) Mn-Cr-W system, which is a representative steel which has good comprehensive performance in all countries and is represented by CrMn. This steel has the characteristics of small heat treatment deformation, high wear resistance, high hardenability and toughness. The mold made of CrWMn steel, such as the use of reasonable heat treatment process, control and adjustment, the thermal stress and the tissue stress caused by the quenching, and reduce the deformation can completely replace the common Cr12 blanking die made by the factory.

    2) Mn system, 9Mn2V is China's substitute for CrWMn, 9CrSi and so on. It can be used to make cold punching die, blanking die, scissors, bending die.

    3) SiMnMo line graphite steel is characterized by high Si content. Under the action of strong graphitization element Si, some carbon is in the state of graphitization, which makes the steel self moistening, has high anti bite resistance, good cutting property and wear resistance, heat treatment deformation is small, and the size is stable. SiMnMo steel is used for making bending, stretching and shaping die with strict surface quality, which can significantly improve the quality of stamping parts.

    2.3 the high wear resistant micro deformation cold formed die steel is represented by Cr12Mov type, which is the most widely used cold deformation die steel at home and abroad. Cr12Mov is an improved Cr12 steel with carbon content over

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