Design considerations for the third generation advanced high strength steel

Abstract

Advanced high strength steels containing metastable austenite are of considerable interest due to the combinations of strength and ductility that are achieved via austenite transformation to martensite during deformation. A methodology is presented to design microstructures containing systematic amounts of metastable austenite with controlled stability against transformation based on Mn enrichment of austenite during intercritical annealing of medium Mn (5 to 10 wt pct.) low carbon (0.1 and 0.15 wt pct.) steels. Five steels were selected for experimental investigation. Three cold rolled low C (0.1 wt pct. C) medium Mn-TRIP steels (5.1, 5.8, and 7.1 wt pct. Mn) steels were annealed at temperatures between 575 °C and 675 °C for 168 hr to enrich austenite in C and Mn. The predicted amount of Mn in austenite decreased from 14.7 wt pct. (575 °C) to 8.5 wt pct. (675 °C) in the 7.1Mn-0.1C steel. The heat treated microstructures consisted of ferrite, [epsilon] and [alpha]' martensite and austenite amounts between 0 and 47.5 pct. Two low C (0.14 wt. pct.) medium Mn (7.4 and 10.1 wt pct.) high Al (1.6 wt pct.) steels were annealed using a two-step method, intercritical annealing at 600 °C or 700 °C for 96 hr followed by cold rolling and supercritical annealing at 850 °C to produce martensitic microstructures with retained austenite. Uniaxial tensile testing, stress relaxation testing, and electron microscopy were used to characterize microstructural changes with deformation; in situ neutron diffraction was also performed on selected steels. The intercritically annealed 0.1C Mn-TRIP steels displayed systematic changes in tensile behavior dependent on the intercritical annealing temperature; ranging from high-ductility limited work hardening for the lowest test temperature (575 °C, 32.6 wt pct. austenite), to increasing strain hardening resulting in high strength and ductility (600 °C, 38.8 wt pct. austenite) to low ductility, high strength at the highest annealing temperatures (650 °C, 47.5 wt pct.). The stability of austenite during deformation depended on the C and Mn content of austenite, lower Mn contents corresponded to rapid transformation at yielding (650 °C) while high Mn contents resulted in limited austenite transformation (575 °C); optimum tensile properties resulted from significant austenite transformation above 10 pct. strain (600 °C). Heat treatment of the medium Mn-0.14C-1.6Al steels resulted in martensitic microstructures with alloy and processing dependent amounts of retained austenite, 8.5 wt pct. in the 7.4Mn-0.14C-1.6Al steel and 22 wt pct. in the 10.1Mn-0.14C-1.6Al steel. The 7.4Mn-0.14C-1.6Al steel showed power law hardening after austenite transformation resulting in plastic instability while the 10.1Mn-0.14C-1.6Al steel displayed apparent brittle fracture during uniform deformation. The change in tensile behavior corresponded to a change in the distribution of internal stresses between phases with austenite transformation. Austenite morphology is suggested to control the distribution of internal stresses in the microstructure.