Cold work tool steels are the main raw materials for tools such as gauge, drill steel and punch press, as well as dies such as stamping, forging, rolling and rolling. In cutting, bending, stamping, compression and other processes, the mold is subjected to high surface pressure and impact load, most of the mold due to wear, hot bending, cracking and other reasons scrapped. Cold work tool steels generally need to be quenched and tempered and used under the condition of the hardness of about 60HRC. Therefore, high strength, high wear resistance, high fatigue resistance and high toughness are required.
Wear resistance is the most important characteristic of cold work tool steels. The basic method to improve wear resistance is to increase the hardness of steel. Increasing the content of C in steel can improve the hardness. In addition, the addition of Cr, Mo, V, W and other elements can make the steel contain M7C3, M6C, MC and other hard carbides, which can further improve the wear resistance of steel. However, the toughness and fatigue resistance of steel decrease with an excessive amount of carbide. Therefore, the suitable cold work tool steels should be selected according to their uses.
The JIS standard mainly includes carbon tool steels and alloy tool steels, of which 11 are carbon tool steels and 10 are alloy tool steels. The C content of the carbon tool steel is 0.55% ~ 1.50% and contains a little Si and Mn. After quenching at 750 ~ 840℃ and tempering at 150 ~ 200℃, its hardness can reach 54 ~ 63HRC. Because there are few alloying elements in carbon tool steel and poor hardenability, it is necessary to carry out water cooling quenching, but only the surface is hardened while the center hardness is still low. The most commonly used grade is SK105 (1.05C-0.2Si-0.25Mn), which can be used as cutting tools, drill steel, jig and punching die. JIS classifies alloy tool steels as cutting tool steels, impact-resistant tool steels, cold work die steels, hot work die steels, and many grades of improved steels. The following is a brief description of the characteristics of the representative grades of alloy tool steels.
Low alloy tool steels
Low alloy tool steels contain small amounts of alloying elements in cold work die steels. The commonly used grades are SKS93 (1.05C-0.95Mn-0.4Cr) and SKS3 (0.95C-1.05Mn-0.75Cr). SKS3 steel also contains 0.75% W which improved hardenability and toughness. It can be used to make cutting edges, gauges, punching dies, etc.
High C-Cr cold work tool steel
The typical material is SKD11 (1.5C-12CR-1MO series). The high Cr content produces the coarse Cr carbide (M7C3) in the parent phase, which improves its wear resistance. SKD11 has good hardenability, and uniform core hardness can be obtained by air cooling. The quenching temperature is 1000 ~ 1040℃, which is commonly used in vacuum heat treatment furnaces.
The cold work tool steel with better wear resistance than SKD11 is SKD1 (2C-12Cr series), which has more C and Cr content, making SKD1 tool steel with high wear resistance. However, the large amount of Cr carbide makes it less toughness and fatigue resistant than SKD11. Because the quenching temperature of SKD1 is 950℃, which is different from the commonly used quenching temperature and has poor hardenability, it must be oil quenched, so SKD1 is not commonly used. A variety of improved steels have been developed on the basis of SKD11 for different purposes and have been widely used.
5%Cr cold work tool steels
The typical grade of JIS 5%Cr cold work tool steels is SKD12 (1C-5Cr-1Mo series).SKD12 has excellent toughness, less C and Cr content than SKD11, but slightly poor wear resistance. The advantage of SKD12 is that the heat treatment size deformation anisotropy is small. The quenching temperature of SKD12 is 970℃, the tempering hardness is 60HRC at low temperature, and the tempering hardness is less than 58HRC at high temperature. These are different from general tool steels, so they are rarely used.
8%Cr cold work tool steel
Although 8%Cr steel is not included in the JIS standard, it is widely used. The steel adjusted element content based on SKD11 and the main composition is 1C-8Cr-2Mo. Mold steels often crack because of EDM and quenching residual stress and insufficient toughness. High-temperature tempering can effectively eliminate residual stress, but SKD11 after high temperature tempering can not get the required hardness and toughness.
Therefore, reducing C and Cr and increase Mo on the basis of SKD11 to form 8%Cr steel, make steel by high temperature tempering can also obtain 62HRC high hardness. 8%Cr is not as good as SKD11 in wear resistance and cutting ability. However, due to its excellent comprehensive performance, it is still widely used. It is the standard grade of cold work tool steel with SKD11.
Matrix stamping mold steel
Matrix stamping mold steel is equivalent to matrix steel that eliminating SKD11 coarse carbides. It has not yet been included in the JIS standard but has been developed and widely used by many enterprises. On the basis of 8%Cr, C and Cr are further reduced, so that there is almost no coarse carbide in the steel, so the toughness and fatigue resistance of the steel is much better than that of 8%Cr steel. The machinability and anisotropy of heat treatment dimension deformation are also improved. Matrix stamping mold steel shortens the manufacturing time of finished mold.
Flame surface hardened steel
Tool steels are generally used after conventional quenching and tempering. Flame surface quenched steel is simplified quenching and tempering treatment of steel, the main component is 0.7C-1Si-1Mn-1Cr, is the processed mold hardness requirements of heating parts with welding nozzle or fuse nozzle, and then cooling, to obtain hardness above 60HRC. Flame surface hardened steel is characterized by a wide quenching temperature range and has a certain hardenability, toughness and weldability. It is not included in the JIS standard, but because the price is cheaper than stamping dies steel, and less heat treatment hours, short delivery time, gas been widely for cutting die in automobile manufacturers.