INTERCRITICALLY ANNEALED AND ISOTHERMALLY TRANSFORMED 0.15-PCT-C STEELS CONTAINING 1.2-PCT SI-1.5-PCT MN AND 4-PCT NI .2. EFFECT OF TESTING TEMPERATURE ON STRESS-STRAIN BEHAVIOR AND DEFORMATION-INDUCED AUSTENITE TRANSFORMATION

Stress-strain behavior and deformation-induced transformation of retained austenite were studied for intercriticaly annealed and isothermally transformed Si-Mn and Ni steels as a function of testing temperature between -80-degrees-C and 120-degrees-C. Rapid decrease of retained austenite at small strains dominates at low-temperature testing and in microstructures containing martensite. The austenite transformation in microstructures without martensite shifts to larger strains with increasing testing temperature. The accompanying increase of strain-hardening rates at larger strains deters the onset of necking and improves ductility. The benefits of the austenite transformation lead to a peak in ductility between 20-degrees-C and 70-degrees-C in the Si-Mn steel and at 70-degrees-C in the Ni steel. The peaks are dependent on the nature of the dispersed microconstituents produced in the ferrite during isothermal transformation. Higher testing temperatures enhance the mechanical stability of the austenite and result in lower ductility.