Process characteristics of die steel and its classification and grade

Definition of steel

Steel is an iron-carbon alloy with a carbon content of 0.0218% to 2.11%. In ordinary steel, add Cr, Mo, V, Ni, and other alloy components to get alloy steel, our mold steel is alloy steel.

Ways to change the performance of steel

Alloy composition

(1) Carbon: C

Role: increase the hardness of the quenching organization; the formation of carbide, improve wear resistance; reduce toughness; reduce weldability.

(2) Chromium: Cr

Role: improve the hardness of steel, the formation of hard and stable chromium carbide, thus improving wear resistance; can improve the hardenability of steel; when the Cr content of more than 12%, with corrosion resistance, and provide a good throw spin.

(3) Molybdenum: Mo

Role: Mo is a strong carbide forming element, improving wear resistance; Mo>0.5% can suppress other alloying elements caused by tempering brittleness; improve red hardness, and thermal strength; improve hardenability, and tempering stability.

(4) Vanadium: V

Role: can form high hardness carbide, improve wear resistance; refine the grain of the steel, reduce superheat sensitivity; improve the strength, toughness, and tempering stability of steel.

(5) Nickel: Ni

Role: Ni can improve the hardenability of steel; Ni can refine the grain.

(6) Sulfur: S

Role: often in the form of MnS steel, cutting the continuity of the matrix, deteriorating the toughness of the material, corrosion resistance, throwing spin, electrical discharge processing, and etching, can improve the cutting ability of the material.

Smelting process

(1) Ordinary steelmaking process
(2) Electroslag remelting (ESR)

The crude steel billet is placed in the electric slag furnace, and a strong current is passed through the furnace to produce a very high temperature so that the crude steel billet melts into steel, and the steel flows through the electric slag.

The impurities are filtered and adsorbed by the electric slag, thus achieving the effect of purification. The overall remelting speed is fast, but some very fine impurities are not removed.

(3) Vacuum arc remelting (VAR)

In the vacuum furnace, the strong current is passed on the billet, the bottom of the billet begins to melt, and the impurities vaporize into gas and are pumped away.

Thus increasing the purity of the steel, and it solidifies drop by drop, the solidification speed is very fast, and the tissue becomes very dense. The characteristic is that the impurities are completely removed, but the overall remelting speed is slow.

Heat treatment

Heat treatment of steel refers to the process of heating and then cooling to change the properties of steel by controlling the heating temperature, holding time, and cooling rate of steel to meet the processing or use requirements.

The main heat treatment processes are annealing, normalizing, quenching, and tempering.

Classification of die steel by use

Cold work die steel

Cold work dies steel, is mainly used in the manufacture of the cold state of the workpiece for the press molding mold. Such as cold punching dies, cold stamping dies, cold drawing dies, embossing dies, cold extrusion dies, thread pressing dies and powder pressing dies, etc.

The range of cold work tool steel is very wide, from various carbon tool steel, alloy tool steel, and high-speed tool steel to powder high-speed tool steel and powder high alloy tool steel, etc.

Hot work tool steel

Hot work dies steel, is mainly used in the manufacture of high-temperature state of the workpiece for pressure processing of the die. Such as hot forging dies, hot extrusion dies, die-casting dies, hot forging dies, etc.

Commonly used hot work die steel are: high carbon content added Cr, W, Mo, V and other alloying elements of alloy die steel; special requirements of hot work die steel, sometimes using high alloy austenite heat-resistant die steel manufacturing.

Plastic mold steel

Due to the many varieties of plastics, plastic product requirements also vary greatly, and the manufacture of plastic mold materials also put forward a variety of different performance requirements.

Therefore, many industrial countries have formed a wide range of plastic mold steel series, including carbon structural steel, carburizing plastic injection mold steel, pre-hardening plastic mold steel, age-hardening plastic mold steel, corrosion-resistant plastic mold steel, easy-to-cut plastic mold steel, overall hardening plastic mold steel, maraging steel and mirror polishing with plastic mold steel, etc.

Requirements of steel for moulds

Generally the mold according to the length of service life into five levels: one in a million times, two is 500,000 – 1 million times, three in 300,000 – 500,000 times, four in 100,000 – 300,000 times, and five in 100,000 times or less.

First-class and second-class injection molds are required to use heat treatment, hardness in the HRC50 steel, otherwise easy to wear, injection molding products easy to super poor, so the selected steel should have good heat treatment performance, but also in the state of high hardness has a good cutting performance, of course, there are other considerations.

Usually choose Sweden’s 8407, S136, the United States 420, H13, Europe’s 2316, 2344, 083, or Japan’s SKD61, DC53 (originally for hardware mold materials, special circumstances used).

There is strong corrosive plastic generally choosing S136, 2316, 420 and other steel, weak corrosive in addition to the choice of S136, 2316, 420, SKD61, NAK80, PAK90, 718M.

The appearance requirements of the product also have a great influence on the choice of injection mold materials. Transparent parts and the surface requirements of mirror products, the materials available are S136, 2316, 718S, NAK80, PAK90, 420, the transparency of the mold should be selected S136, followed by 420.

The third level mold with pre-hardened material, the grades are S136H, 2316H, 718H, 083H, hardness HB270~340.

Fourth and fifth level molds with P20, 718, 738, 618, 2311, 2711, for the requirements of particularly low mold, there may be used S50C, 45 # steel, that is, directly in the mold embryo to do the cavity.