Effects of Cooling Rate in an Innovative Heat Treatment Route for High-Strength Steels

The requirement for high strength and ductility is usually associated with martensitic microstructure with a certain amount of retained austenite. One of the innovative heat treatment processes that can lead to such microstructure is the Q&P process (Quenching and Partitioning). It can produce microstructures consisting of martensite and a certain amount of retained austenite, which exhibit strengths above 2000 MPa and elongation levels of more than 10%. The objective of this research was to explore the effects of the cooling rate in the Q&P process and evaluate the effects of various microstructure constituents on mechanical properties of high-strength steels. Three newly-created experimental steels, which contained 0.43% carbon and had reduced Ms temperatures thanks to manganese addition, were subjected to several heat treatment routes which involved various cooling rates. The cooling rates were chosen on the basis of calculations using the JMatPro software and earlier results. It was found that by varying the cooling rate one can obtain various mixed microstructures and a wide range of mechanical properties. The strengths were in the range of 1200-2300 MPa and A5mm elongation levels were up to 18%. Because the amount of retained austenite has a considerable impact on the resulting mechanical properties, it was measured by means of X-ray diffraction.