Dynamic continuous cooling transformation of Nb microalloyed NM500 low-alloy high-strength wear-resistant steel

Nb microalloyed NM500 low-alloy high-strength wear-resistant steel is a new generation of steel developed and widely used in the machinery industry nowadays. It has the advantages of high yield strength, excellent plasticity, high toughness and wear resistance, good weldability, rust resistance and fine workability, etc. The microstructure of the steel consists of martensite, bainite and retained austenite, and so on. In order to more effectively improve the strength/toughness balance of the steel and achieve the required mechanical properties, the dynamic continuous cooling transformation process of the Nb microalloyed NM500 steel was studied.
    
The dynamic continuous cooling transformation process of Nb microalloyed NM500 steel is a process that can rapidly transform the microstructure of Nb microalloyed NM500 steel and improve the mechanical properties. The specific process of the dynamic continuous cooling transformation of the Nb microalloyed NM500 steel is as follows (Figure 1):
    
First, the as-cast single-stage Nb microalloyed NM500 steel is heated to austenitizing temperature. Then, the steel is quenched in a cooling medium, such as water or oil, to rapidly cool it. After the cooling process is completed, the steel is immediately subjected to dynamic continuous cooling transformation. The dynamic continuous cooling transformation process is usually comprised of three stages: rapid cooling, controlled cooling and low-temperature tempering. During rapid cooling, the heating rate of the Nb microalloyed NM500 steel is rapidly decreased to below the critical cooling rate. The critical cooling rate of the steel is usually 10–40 °C/s. At this cooling rate, the steel can be cooled to the Ms temperature, at which, the austenite is fully transformed into martensite. During the controlled cooling, the cooling rate of the steel is decreased and maintained around 0.3–7 °C/s, and the cooling temperature is controlled to a point slightly below the Ac1 temperature. At this temperature, austenite can start to transform into bainite. During the low-temperature tempering, the cooling rate of the steel is decreased to very low and the steel is held for some time in the temperature range of 150–450 °C to further transform martensite, bainite and austenite into the optimal microstructure.
    
In brief, the dynamic continuous cooling transformation process of Nb microalloyed NM500 steel is a more effective process which can vastly improve the mechanical and wear-resistant properties of the steel due to the formation of an appropriate microstructure. It has the advantages of shorting production cycle time, saving energy and reducing environmental pollution. It has been applied successfully in many parts of the machinery industry to produce wear-resistant products, such as excavator parts, bicycle frames, construction parts and heavy machinery parts, etc.

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