5. Carburizing, nitriding, boronizing and metalizing treatment

Chemical heat treatment can effectively improve the wear resistance, corrosion resistance, seizure resistance and oxidation resistance of the mold surface. Almost all chemical heat treatment processes can be used for the surface treatment of mold steel.

Research work shows that both high carbon and low alloy tool steels and medium and high carbon high alloy steels can be carburized or carbonitrided. When carburizing or carbonitriding of high carbon low alloy steel, the lower heating temperature and shorter holding time should be selected as much as possible. At this time, the surface layer has more undissolved carbide core, carburizing and carbon nitrogen. After co-infiltration, the surface carbides are in the form of particles, and the total volume of carbides is also significantly increased, which can increase the wear resistance of the steel. After the carburizing of W6Mo5Cr4V2 and 65Nb steel molds and vacuum carburizing of 65Nb steel molds, the life of the molds was significantly improved.

Alloy steel molds with high temperature tempering at 500-650 °C can be surface nitriding or nitrocarburizing in the range below tempering temperature or tempering.

The nitriding process currently employs processes such as ion nitriding and high frequency nitriding. Ion nitriding can shorten the nitriding time and obtain a high quality permeable layer. In addition, ion nitriding can also improve the corrosion resistance, wear resistance, thermal fatigue resistance and adhesion resistance of the die-casting mold.

Nitrocarburizing can be carried out in a gaseous medium or a liquid medium, the permeable layer is less brittle, and the osmosis time is much shorter than the nitriding time. Die-casting molds and hot extrusion dies can significantly improve their thermal fatigue properties after nitrocarburizing. Nitrocarburizing has a good application effect on cold heading die, cold extrusion die, cold die and drawing die.

Cold work dies and hot work dies can also be sulphur nitrogen or sulphur nitrocarburizing. In recent years, many research work has shown that rare earths have obvious percolation effects, and new technologies such as rare earth nitrogen co-infiltration and rare earth nitrocarburizing have been developed.

The nitriding treatment of the mold includes ion nitriding and gas nitriding. The hot boring die uses ion nitriding, which can significantly improve the service life of the mold. The die-casting mold can adopt gas nitriding, and the corrosion resistance after nitriding is greatly enhanced.

Boronizing can be solid boronizing, liquid boronizing and paste boronizing. The most widely used is solid boronizing, and solid boronizing agent is available on the market. After solid boronizing, the hardness of the surface layer is as high as 1400-2800 HV, and the wear resistance is high, and the corrosion resistance and oxidation resistance are good.

The boronizing process is often used on various cold working dies, and the life of the mold can be increased several times or more than ten times due to the improvement of wear resistance. Boronizing with medium carbon steel can sometimes replace high alloy steel to make molds. Boronizing can also be applied to hot working molds, such as hot extrusion dies.

The boronized layer is brittle, the diffusion layer is relatively thin, and the supporting force to the infiltration layer is weak. For this reason, boron nitrogen co-infiltration or boron carbonitriding can be used to strengthen the transition zone and make the hardness change gentle. In order to improve the brittleness of the boronized layer, boron vanadium and boron aluminum can be used for co-infiltration.

The metal infiltration includes chromizing, vanadium infiltration and titanium infiltration. It can be used to treat cold work and hot work molds. The TD method (molten salt metallization) has been used to increase the life of the mold by several times or even ten times. .

3. Prospects and recommendations for automotive mold manufacturing

In the context of today's economic globalization, mold materials and heat treatment have flourished. The development of a technology is quickly being applied to all parts of the world. With the improvement of automotive technology, it is inevitable to improve the manufacturing level of automotive molds. In the future, the development of automotive mold materials and advanced heat treatment technology will also be greatly developed.

1. The development of die steel production towards the commercialization of product, fine material and die steel distribution

During the "Tenth Five-Year Plan" period, China's annual mold steel used more than 300,000 tons, of which 150,000 tons are steel castings; 150,000 tons are round steel, sheet and block materials. In recent years, the import volume of foreign die steel has been decreasing year by year, and the export volume of our die steel is increasing. The degree of productization, fine materialization and distribution of mold steel production in China is gradually increasing. In some industrialized countries, metallurgical enterprises have supplied 50% to 60% of machined die steel products, and China has gradually improved the supply of die steel to black leather round bars. Because more and more mold manufacturers require mold steel suppliers to quickly supply the required steel after the mold design is completed, reduce the amount of steel in stock, and shorten the production cycle of mold manufacturing. In order to adapt to this commodity market mechanism, domestic steel mills and other production enterprises gradually provide sheet metal mold and block mold steel to mold manufacturers, which is the result of market economy development.

2. Vigorously promote the application of new mold steel with excellent performance, and constantly improve the mold steel series

At present, a number of new mold steels with certain characteristics have been developed, some of which have excellent performance and can reach or exceed the level of similar steel at a very low cost. The number and application range of these new steel grades are very large. These new mold steels with good performance have broad prospects for promotion and will generate huge economic benefits.

3. Further improve the quality of mold materials

Some special steel mills in China have used new metallurgical equipment and processes to produce mold steel, such as furnace refining, vacuum smelting, fast forging machines and precision forging machines. The quality of some mold steels has been greatly improved, such as D2, P20, etc. Steel has been exported in batches, and the quality of exported products can reach the international advanced level. Industrial developed countries have been working hard to improve the purity, density, uniformity and quality stability of die steel. Some foreign companies stipulate that [O]≤1×10-5, [H]≤2×10-6,[S]≤5×10-5 in high-purity die steel, because the purity of steel can be further improved. Significantly improve the toughness and fatigue properties of steel. For large molds, vacuum degassing and high temperature diffusion annealing must also be used to reduce the segregation of alloying elements and use an isotropic forging process to improve the isotropic properties, so that the transverse and thick plasticity and toughness of the die steel can reach 80% of the longitudinal direction. ~90% or more.

4. Strengthen the promotion and application of advanced mold heat treatment technology

The controlled atmosphere heat treatment and vacuum heat treatment of the mold should be further developed, promoted and applied. For some effective mold surface heat treatment techniques, the process should be improved and its promotion and application, such as induction hardening and laser quenching technology, should be strengthened. Improve the manufacturing level of equipment and process materials, and strengthen the construction of professional heat treatment plants.

5. Vigorously promote the casting and forging process

In some fields, such as hot forging die and cold die, casting and forging can save raw material consumption, material isotropic, reduce CNC machining cost, shorten production cycle and reduce manufacturing cost. It has broad social benefits for resource conservation. Replace the steel with cast iron; replace the 4Cr2MoVNi with an alloy cast steel as the machine forging die; replace the forged Cr12Mo1V1 or Cr12MoV with the cast Cr12Mo1V1 steel (1.2382 in Germany). The latter has been well applied to Volkswagen's automotive cover moulds. Of course, due to the high carbon content in high-alloy steel, the presence of reticulated carbides makes the inserts prone to cracks during heat treatment, which requires strict steel casting and heat treatment processes, and the entire process must always control grain boundary impurities. At a lower level. We recommend the promotion of casting and forging technology.

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