The mould steel plate is a type of steel used to make a cold-punching mould, hot forging mould, die-casting mould and other moulds, the performance of which is determined by the following factors:
1. Strength properties
1) Hardness
Hardness is the main technical indicator of mould steel plate, the mould must have a sufficiently high hardness to maintain its shape and size under the action of high stress. For the same type of steel, the hardness is proportional to the deformation resistance within a certain hardness value range. However, there may be obvious differences in plastic deformation resistance between steels with the same hardness value but different compositions and structures.
2) Red hardness
Hot work molds that work at high temperatures require the stability of their structure and performance to maintain a sufficiently high hardness. This performance is called red hardness. The red hardness of steel mainly depends on the chemical composition and heat treatment process of steel.
3) Compressive yield strength and compressive bending strength
The mould is often subjected to high-strength pressure and bending in the process of using. Therefore, the mould material is required to have a certain compressive strength and bending strength. In many cases, the conditions for the compression test and the bending test are close to the actual working conditions of the mould.
2. Toughness
In the working process, the mould is subjected to impulsive load, so in order to reduce the damage in the form of breakage and tipping during use, the mould steel plate is required to have a certain degree of toughness.
The number, morphology, size and distribution of the chemical composition, grain size, purity, carbides and inclusions of the mould steel plate, as well as the heat treatment system of the mould steel plate and the metallographic structure obtained after heat treatment and other factors all have a great influence on the toughness of steel. The toughness, strength and wear resistance of steel are often contradictory. Therefore, it is necessary to choose the chemical composition of steel rationally and adopt the reasonable refining, thermal processing and heat treatment technology, so that the best combination of abrasive resistance, strength and toughness of the mould material can be achieved.
3. Abrasive resistance
The mould is subjected to considerable compressive stress and friction in the course of work, and it is required that the mould can maintain its dimensional accuracy under the strong friction. In order to improve the abrasive resistance of the mould steel plate, it is necessary not only to maintain the high hardness of the mould steel plate but also to ensure that the composition, morphology and distribution of carbides or other hardened phases in the steel are reasonable.
4. Thermal fatigue resistance
In addition to the periodic variation of the load, the hot work mould steel plate is also subject to high temperature and periodic extreme cold and extreme heat under service conditions. Therefore, the evaluation of the fracture resistance of the hot work mould steel plate should pay attention to the thermal-mechanical fatigue fracture performance of the material.
5. Occlusive resistance
The occlusive resistance is actually the resistance when "cold welding" occurs. This performance is more important for mould materials. During the test, usually under the dry friction conditions, the tested tool steel sample and the material with an occlusive tendency (such as austenitic steel) are subjected to constant-speed dual friction movement, and the load is gradually increased at a certain speed. At this time, the torque also increases correspondingly, this load is called "occlusive critical load", and the higher the critical load marks the stronger the occlusive resistance.
