Application of direct resistance treatment to localized areas on advanced high strength steel sheet

Abstract

The possibility of creating local-designed variations in microstructure has the potential to radically change how advanced and ultra-high strength steels are used in vehicle components. Such local changes in the microstructure offer the potential for tailored combinations of strength and ductility, both reducing vehicle weight and improving structural performance. This study addresses development of a new technology for creating such local changes in microstructure. The process, termed direct resistance heat treatment (DHRT) is analogous to resistance welding with in situ tempering. Current impulses are used to first locally transform the microstructure (austenitize and quench to martensite) then temper the martensitic microstructure to achieve desired levels of performance. Work here was done on a 1-mm thick 10B21 steel. Practices were first developed to achieve fully martensitic microstructures, and then combinations of temper currents and times were used to achieve differing final hardnesses. Thermal analyses suggested that for the material studied, stable temperatures (during heating) could be achieved in about 50 ms, and the workpiece could effectively quenched in about 100 ms. Mechanical evaluations showed that the locally hardened areas were effective for diverting plastic deformation, even when using notched specimens.