Performance of die steel
Strength property
(1) Hardness is the main technical index of die steel. The mold must have high enough hardness if it wants to keep its shape and size unchanged under the action of high stress. The general hardness of cold working die steel at room temperature is kept at about HRC60, while the hardness of hot working die steel is generally kept in the range of HRC40~55 according to its working conditions. For the same steel, the hardness is proportional to the deformation resistance in a certain range of hardness values. However, the plastic deformation resistance of steel with the same hardness value and different composition and structure may be significantly different.
(2) Red hard hot working die working at high temperature requires to maintain the stability of its structure and performance, so as to maintain a high enough hardness. This performance is called red hard. Carbon tool steel and low alloy tool steel usually maintain this property in the temperature range of 180~250℃, and chrome-molybdenum hot working die steel generally maintains this property in the temperature range of 550~600℃. The hardness of steel depends mainly on its chemical composition and heat treatment process.
(3) Compressive yield strength and compressive bending strength The die is often subjected to high strength pressure and bending in the process of use, so it is required that the die material should have a certain compressive strength and bending strength. In many cases, the conditions of the compression and bending tests are close to the actual working conditions of the die (for example, the measured compressive yield strength of the die steel is in good agreement with the deformation resistance shown by the punch during operation). Another advantage of bending test is that the absolute value of the strain variable is large, which can sensitively reflect the difference of deformation resistance between different kinds of steel and under different heat treatment and microstructure conditions.
toughness
In the process of working, the die is under impact load. In order to reduce the damage in the form of breaking and falling blade in the process of use, the die steel is required to have certain toughness.
The chemical composition of die steel, grain size, purity, quantity, morphology, size and distribution of carbide and inclusions, as well as the heat treatment system of die steel and the metallographic structure obtained after heat treatment have great influence on the toughness of steel. In particular, the influence of steel purity and hot working deformation on its lateral toughness is more obvious. The toughness, strength and wear resistance of steel are often contradictory. Therefore, it is necessary to choose the chemical composition of steel reasonably and adopt reasonable refining, hot processing and heat treatment technology, so as to make the wear resistance, strength and toughness of die materials reach the best match.
Impact toughness refers to the total energy absorbed by the specimen during the whole fracture process during a single impact. However, many tools fracture under different working conditions, so the conventional impact toughness can not fully reflect the fracture performance of die steel. Test techniques such as low energy multiple impact fracture work or multiple fracture life and fatigue life are being used.
