Effect of Intercritical Annealing Temperature on the Tensile Behavior of JIS-SAPH440 Steel

Ferritic-Martensitic dual-phase steels are widely used in car manufacturing industries because of their attractive mechanical properties like significant tensile strength and good formability. It is clear that the mechanical properties of these steels are dependent on their microstructural characteristics like the fraction of different phases and the dislocation density of each phase, and these characteristics are affected by the thermomechanical history.  The aim of this work is to investigate the effect of the microstructure of the Ferritic-Martensitic SAPH440 steel on its tensile behavior like the tensile strength and the strain rate sensitivity of flow stress. For this purpose, the steel is subjected to inter-critical annealing at different temperatures between 750 ºC to 790 ºC and then, it is subjected to the tension test at room temperature using two different strain rates. Also, the microstructure evolutions of this steel are studied using optical microscopy, scanning electron microscopy, and X-ray diffraction. Results show that the increase of the intercritical annealing temperature from 750 ºC to 790 ºC causes an increase of the martensite fraction from 31% to 45% and the increase of dislocation density of ferrite from 5×1014 to 1.54×1016. These variations cause the increase of the tensile strength from 800 MPa to 1100 MPa while the strain rate sensitivity of the flow stress decreases to a negligible amount.