Determination of transformation temperatures of advanced high-strength steels and their use in designing Q & P process routes

Abstract

Determining transformation temperatures of novel steels is an important step towards finding parameters for their heat treatment. In advanced high-strength steels for Q & P processing (Quenching and Partitioning), the crucial processing characteristics are the temperatures of the start and end of austenitization and the Ms temperature. Q & P processing is characterized by quenching from a full-austenitization temperature to below the Ms, and subsequent holding at the partitioning temperature. This leads to martensitic microstructures with retained austenite between martensite needles and to ultimate strengths above 2000 MPa and elongation levels up to 10%. Several AHS steels containing 0.4% C were manufactured and cast for this experiment. Their main alloying additions were manganese, silicon, chromium, molybdenum and nickel. Their transformation temperatures were first calculated using the JMatPro software. The values were validated by dilatometry measurements. Based on these results, a Q & P process route was designed and put to test. The resulting microstructures were documented using optical and scanning electron microscopy. Strengths of more than 2300 MPa and up to 11% elongation levels were obtained.