Since the profile size of the hot pressing forming mold for the ultra-high strength steel plate of the automobile is different and the shape is complicated, there are a large number of grooves, corners, protrusions and concavities, and the requirements for dimensional accuracy and surface accuracy are high. If the processing is carried out after quenching, chipping and cracking will occur, and only by quenching as a whole and then processing can such phenomena be avoided.

How to process the hot pressing forming mold of automotive ultra-high strength steel plate is a difficult point, and high-speed milling is inevitably one of the directions in the processing of automotive ultra-high strength steel plate hot pressing forming mold. In the following, a simple discussion is made from three aspects: the selection of automotive ultra-high strength steel plate hot pressing forming mold material, the quenching of hot pressing forming mold material and the processing of hot pressing forming mold material.

The service life of the mold directly affects the cost of the parts. When designing the hot pressing forming mold for automotive ultra-high strength steel plates, different materials must be selected according to different usage conditions to ensure the life of the mold.

After the production batch is determined, many factors need to be considered when selecting materials in the process of mold design, such as: material hardness, wear resistance, toughness (ductility), dimensional stability, machinability, wire cutting machining, surfacing welding, polishing, quenching, heat hardness, impact resistance, cutting and shearing performance, etc. In the field of hot pressing molds for ultra-high strength steel plates for automobiles, cracks and wear resistance are even more important, and the requirements are very high. Therefore, the limit of strength, wear resistance and toughness plays a key role in the life of the mold, and is also one of the main factors to consider when selecting mold materials.

The mold material is mostly die steel such as Cr12MoV and 5CrMnMo (hardness of 55～65 HRC), which belong to high-strength and high-hardness materials. Moreover, most of them are assembled by inlaid hardening steel and then milled as a whole, and there is a large proportion of non-uniform high-hardness surface (hardness of 55～65 HRC) in the machining area.

The wear resistance of the hot pressing forming mold for the ultra-high strength steel plate of the automobile directly affects the service life of the mold, so the hardness and wear resistance of the working parts such as the mold convex and concave die are improved by quenching, which can directly extend the service life of the mold. The material with these treatments has a more uniform and dense metal organization, and the strength and toughness are improved, and it has better comprehensive mechanical properties. At the same time, the workpiece after these treatments will also have different degrees of deformation. Designers should consider the hardenability of the material and the machining after quenching, tool wear, die deformation, machining amount, mold processing cycle, CNC and fitter workload and other factors when designing the need for overall quenching insert.

Domestic automotive mold enterprises usually adopt two methods to process hard steel molds: one is the traditional grinding processing adopted by most enterprises; the other is the high-speed hard milling processing adopted by a few enterprises.

Compared with traditional grinding process, high-speed hard milling process has the advantages of good process flexibility, no pollution and high production efficiency, so, high-speed hard milling process of automotive quenched steel mold must become one of the key technologies to be studied and developed in the automotive ultra-high strength steel plate hot pressing forming mold manufacturing industry in China. It is of great practical significance to study the manufacturing level of automotive ultra-high strength steel plate hot pressing forming mold and to promote the development of automotive mold industry.

Below, the main aspects of high-speed cutting are briefly discussed: factors of high-speed cutting force, optimization of high-speed cutting methods and selection of high-speed cutting tool materials.

1. Factors affecting the cutting force in hard cutting

Mainly include: cutting speed, cutting depth, feed rate, wear amount of the back tool face and the hardness of the workpiece and so on.

A large number of experimental studies have shown: as the cutting speed increases, the cutting force increases. When the cutting speed exceeds a certain specific value, the cutting force decreases. High-speed milling of quenched steel, the axial force (Z-axis) is far higher than the other two components, which is the main cutting force. Tool wear and tool shape have a great influence on cutting force. As the tool wear intensifies, all the components of cutting force increase. The ground tool edge produces greater cutting force than the chamfered tool edge, and the cutting force increases as the axial cutting depth increases.

2、Optimization of process parameters for high-speed milling of hardened steel molds

Since the high-speed cutting mechanism is different from the traditional cutting mechanism, at present, the high-speed milling process of quenched steel mold can not simply follow the relevant theory and concept of conventional milling. Therefore, some scholars at home and abroad optimize the process parameters of high-speed milling of quenched steel mold, and the main methods are as follows: the method of optimizing the test, the combination of test optimization and algorithm optimization, which mainly takes the cutting speed, feed rate and cutting depth and other main cutting parameters as the design variables, sets the optimization goal, and establishes the simulation model on the basis of the high-speed milling test, and finds a better match of cutting parameters.

The commonly used test method is orthogonal test, and the optimization algorithm is more applied in genetic algorithm, particle swarm optimization algorithm, ant colony optimization algorithm, etc., combined with artificial intelligence methods, and has achieved certain effects in the research of high-speed cutting process parameter optimization.

3、High-speed cuttingToolsMaterial selection

Since the profile size of the hot pressing forming mold for the ultra-high strength steel plate of the automobile is different and the shape is complex, there are a large number of grooves, corners, protrusions and depressions, etc., in order to ensure the surface quality of the quenched steel mold, one process often requires the same tool to complete, so it is required that the tool has high hardness, good heat resistance and chemical stability, and has good impact resistance, wear resistance and other properties.

The main materials of cutting tools are selected as coated cemented carbide, PCBN and ceramic, among which, the main characteristics of ceramic cutting tools are that they have stronger anti-adhesion ability than cemented carbide, and the main characteristics of PCBN are that they have high hardness, good wear resistance and heat resistance, and PCBN has better relative performance in the milling processing of hardened steel molds.

In today's society advocating low-carbon and environmental protection, it is a trend and an important part of mold design to reasonably select mold materials, correctly heat-treat mold materials, use high-speed milling methods to process mold materials, make the service life of molds reach or even exceed the expected service life, and reduce waste. It is inevitable that it will be valued by the majority of mold manufacturing enterprises and application enterprises, and thus widely used in the high-speed milling processing of automotive ultra-high strength steel hot pressing forming molds.