With the development of science and technology in recent years, the technological application in the mold industry has been continuously improved. Mold enterprises have been committed to researching approaches to improving the metallurgical quality of die and mold steel. After years of efforts and exploration, major metallurgical manufacturers at home and abroad are currently focusing on improving the metallurgical quality of die and mold steel in the following aspects:
1. Smelting Quality
For high-quality die and mold steel, processes such as secondary refining, vacuum treatment, vacuum smelting, powder injection, and electroslag remelting (ESR) are widely adopted at home and abroad to reduce the content of harmful elements, oxygen, hydrogen and inclusions in steel, and to fine-tune the chemical composition and casting temperature.
Electroslag remelting can also effectively improve the macrostructure and density of steel, and enhance the isotropy of die and mold steel. Tests by some domestic manufacturers show that the transverse impact toughness of 4Cr5MoSiV1 steel produced by electric arc furnace is only 31% of the longitudinal value. After electroslag remelting, the transverse impact toughness can reach 70% of the longitudinal value, more than doubling the original level.
For die and mold steel with special requirements, powder metallurgy is adopted to produce powder high-speed steel and powder high-alloy die steel, which can better improve the microstructure and properties of steel.
2. Forging and Rolling Processes
On the basis of ensuring a certain forging ratio, upsetting-drawing forging and cross-rolling processes are adopted as much as possible to improve the isotropy of die materials.
To reduce machining allowance and increase material utilization rate, precision forging machines, high-speed forging hydraulic presses and high-precision continuous rolling mills are widely used to produce high-precision steel products to meet the needs of mold manufacturing.
3. Heat Treatment and Finishing
Controlled atmosphere or vacuum heat treatment should be used for the heat treatment of forged and rolled materials to avoid oxidation and decarburization. For some plastic mold steels and hot-work die steels, pre-hardening treatment should be carried out by metallurgical manufacturers.
For some high-performance hot-work die materials, microstructure refinement treatment should be applied by metallurgical manufacturers to eliminate coarse carbides and chain-like distributed carbides in steel, and obtain a fine and uniformly distributed carbide structure, so as to further improve various properties of steel, especially isotropy.
According to some foreign reports, some hot-work die steels processed by electroslag remelting, multi-directional forging (rolling) and microstructure refinement can achieve high quality and high isotropy, with transverse impact toughness reaching more than 90% of the longitudinal value. Many steel plants have registered brand names for steel produced by this process, such as ISODISC by Böhler (Austria), ISOTROPY by Hitachi Metals (Japan), MICROFINE by Kohsei High Speed Steel (Japan), etc. Many domestic metallurgical manufacturers are also committed to this technology.
In addition, considering the metallurgical quality of different parts of steel products, during mold manufacturing, the main working surfaces of the mold (such as the cavity or cutting edge) should be located close to the surface of the steel. Generally speaking, the surface layer of steel is relatively clean, while the core area tends to concentrate macro defects. Especially in large cross-section ledeburite steel, the eutectic carbide inhomogeneity in the central part is 2 to 3 levels higher than that in the surface layer.
Meanwhile, the main load-bearing direction of the mold should be consistent with the deformation direction of the steel to reduce the adverse effect of material anisotropy on the mold.
4. Thermal Conductivity
Thermal conductivity is also one of the key performance indicators of die and mold steel, especially for some hot-work die steels and plastic mold steels. Die steel with high thermal conductivity can quickly dissipate heat generated during processing and transferred from the workpiece, avoiding overheating on the working surface and improving the service conditions of the mold.
For molds for thermoplastic molding and some die-casting molds, in order to speed up production and allow rapid cooling and demolding of pressed parts, mold materials with higher thermal conductivity than steel are sometimes selected, such as high-strength copper alloys and high-strength aluminum alloys.
5. Finished Stock and Productization
To shorten the mold manufacturing cycle, mold manufacturers should preferably select finished stock and products when purchasing die and mold steel, such as finished steel after peeling, cold drawing or grinding, and mold blocks after rough machining, finishing, or even finish machining, quenching and tempering.
With these finished materials and products, mold manufacturers only need minor processing before assembling with standard mold bases. This not only effectively shortens the mold manufacturing cycle to meet market demand, but also reduces production costs and improves material utilization rate, since the production of finished stock and products is carried out efficiently and in large batches in metallurgical plants.
Other Factors for Rational Selection of Die and Mold Steel
When selecting die and mold steel according to the service conditions and requirements of the mold, in addition to the above factors—especially ensuring that the main properties of the steel match the application conditions—the price and universality of the selected steel must also be considered.
Generally, for molds with small size and large production batch, die and mold steel accounts for a small proportion of the total manufacturing cost. Thus, the price of steel may not be a major indicator, and more suitable steel can be chosen.
For large or extra-large molds with simple shapes, since steel cost accounts for a large share of the total mold cost, lower-priced steel can be selected according to production batch. Alternatively, low-cost steel can be used for the mold body, while inserts or surfacing welding with high-performance die steel are applied to key working parts such as the cavity or cutting edge. This method can improve service life while reducing material costs.
The universality of die and mold steel is also an essential consideration. Mold steel is usually used in small quantities but with many varieties and specifications. To facilitate procurement and stock preparation, the universality of materials should be taken into account. Except for special requirements, widely produced general-purpose die and mold steel should be used as much as possible.
General-purpose die steel features mature technology, rich production and application experience, and complete performance data, which provide reliable references for design and manufacturing. In addition, selecting general-purpose die steel facilitates procurement, stock preparation and material management.