Abstract
Austenite stabilization through carbon partitioning from martensite into austenite is an essential aspect of the quenching and partitioning (Q&P) process. Substitutional alloying elements are often included in the chemical composition of Q&P steels to further control the microstructure development by inhibiting carbide precipitation (silicon) and further stabilize austenite (manganese and nickel). However, these elements can interfere in the microstructure development, especially when high partitioning temperatures are considered. In this study, the microstructural development during the Q&P process of four low-carbon, medium-manganese steels with varying contents of silicon and nickel is investigated. During partitioning at 400 °C, silicon hinders cementite precipitation in primary martensite thereby assisting carbon partitioning from martensite to austenite. During partitioning at temperatures of 500 °C and 600 °C, presence of nickel inhibits pearlite formation and promotes austenite reversion, respectively. It is observed that the stabilization of austenite is significantly enhanced through the addition of nickel by slowing down the kinetics of competitive reactions that are stimulated during the partitioning stage. Results of this study provide an understanding of the interplay among carbon, silicon and nickel during Q&P processing that will allow the development of new design strategies to tailor the microstructure of this family of alloys.
Highlights
STEEL grades with high strength and ductility are a paramount requirement for the automotive industry to reduce the consumption of fuel and enhance passenger safety.[1]
For the past few years, special interest has been directed towards the research and development of novel advanced high-strength steels (AHSS) such as quenching and partitioning (Q&P) steels, which have a good combination of high strength and ductility
The phenomena that occurred during the partitioning stage are discussed in detail
Summary
STEEL grades with high strength and ductility are a paramount requirement for the automotive industry to reduce the consumption of fuel and enhance passenger safety.[1]. Speer et al.[2] introduced the Q&P process in which an austenitic microstructure (partial or complete) is quenched to a temperature below the Manuscript submitted October 2, 2020, accepted December 20, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS A martensitic start temperature (Ms) to form a microstructure with a certain fraction of austenite (c) and primary martensite (M1). The process is followed by heating up the steel to a temperature above the initial quenching temperature (TQ) and isothermal holding for a certain time. This is termed the partitioning stage during which carbon partitions from supersaturated martensite to untransformed austenite. The fraction of austenite that is deficient in carbon will transform into fresh martensite (M2)
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