Hot working die steel refers to the alloy tool steel suitable for making dies for hot deformation of metal, such as hot forging die, hot extrusion die, die casting die, hot upsetting die, etc. Because the hot working die has been working under high temperature and high pressure for a long time, the die material is required to have higher strength, hardness and thermal stability, especially high thermal strength, thermal fatigue, toughness and wear resistance.

热作模具在工作时承受着很大的冲击力，模腔和高温金属接触，反复地加热和冷却，其使用条件极其恶劣。为了满足热作模具的使用要求，热作模具钢应具备下列基本特性：

The hot working die bears a great impact force when it works. The mold cavity contacts with the high temperature metal and repeatedly heats and cools, so the working conditions are extremely bad. In order to meet the requirements of hot working die, hot work die steel should have the following basic characteristics:

(1)高的热稳定性。热稳定性是指钢材在高温下可长时间保持其常温力学性能的能力。热作模具工作时，接触的是炽热的金属，甚是液态金属，所以模具表面温度很高，一般为400～700℃。这就要求热作模具钢在高温下不发生热化，具有高的热稳定性，否则模具就会发生塑性变形，造成堆塌而失效。

(1) High thermal stability. Thermal stability refers to the ability of steel to maintain its mechanical properties at room temperature for a long time at high temperature. When hot working die works, it contacts hot metal, even liquid metal, so the surface temperature of die is very high, generally 400-700 ℃. This requires that the hot working die steel does not heat at high temperature and has high thermal stability, otherwise the die will be plastic deformation, resulting in collapse and failure.

(2)优良的耐热疲劳性，热作模具的工作特点是反复受热受冷，模具一时受热膨胀，一时又冷却收缩，形成很大的热应力，而且这种热应力是方向相反，交替产生的。在反复热应力作用下，模具表面会形成网状裂纹(龟裂)，这种现象称为热疲劳，模具因热疲劳而过早地断裂，是热作模具失效的主要原因之一。所以热作模具钢必须要有良好的热疲劳性。

(2) The working characteristic of the hot working die is that it is heated and cooled repeatedly, and the die expands when heated and shrinks at the same time, forming a large thermal stress, and the thermal stress is produced alternately in the opposite direction. Under the action of repeated thermal stress, the surface of die will form network cracks (cracking), which is called thermal fatigue. The premature fracture of die due to thermal fatigue is one of the main reasons for the failure of hot working die. Therefore, hot working die steel must have good thermal fatigue.

(3)较高的高温强度和良好的韧性。热作模具，尤其是热锻模，工作时承受很大的冲击力，而且冲击频率很高，如果模具没有高的强度和良好的韧性，就容易开裂。

(3) High temperature strength and good toughness. The hot working die, especially the hot forging die, bears a great impact force when it works, and the impact frequency is very high. If the die does not have high strength and good toughness, it is easy to crack.

(4)良好的耐磨性能，由于热作模具制作时除受到毛坯变形时产生摩擦磨损之外，还受到高温氧化腐蚀和氧化铁屑的研磨，所以需要热作模具钢有较高的硬度和抗黏附性。

(4) Good wear resistance, because the hot working die is not only subject to friction and wear when the blank is deformed, but also subject to high temperature oxidation corrosion and iron oxide chips grinding, so hot work die steel is required to have high hardness and adhesion resistance.

(5)良好的导热性。为了使模具不致积热过多，导致力学性能下降，要尽可能降低模面温度，减小模具内部的温差，这就要求热作模具钢要有良好的导热件能。

(5) Good thermal conductivity. In order to make the die not accumulate too much heat, leading to the decline of mechanical properties, it is necessary to reduce the die surface temperature and the temperature difference inside the mold as far as possible, which requires the hot working die steel to have good heat conduction performance.

(6)良好的成形加工工艺性能，以满足加工成形的需要。

(6) In order to meet the needs of processing and forming, good forming process performance is needed.

(7)高淬透性。热作模具钢一般尺寸比较大，热锻模尤其是这样，为了使整个模具截面的力学性能均匀，这就要求热作模具钢有高的淬透性能。

(7) High hardenability. Hot work die steel is generally large in size, especially in hot forging die. In order to make the mechanical properties of the whole die section uniform, it requires high hardenability of hot work die steel.